The three types of naturalization

There are three types of naturalization paths. Most are probably going to qualify for 簡易帰化 {kan'i kika} (simplified naturalization) — because most people that apply for permission to naturalize are usually already married to Japanese. Everybody else will fall into the "normal" naturalization process. For all practical purposes, the only major difference between normal & simplified is the amount of time you need to spend in Japan continuously before you qualify for application: one, three, or five years.

Listed from hardest to easiest, they are:

1. 大帰化 {tai-kika} (extraordinary naturalization)
   
   This has never happened in the history of modern democratic Japan, but it's been on the law books since 1950. Simply, it means that you're so full of "win" that both houses — the 衆議院 {shūgiin} (House of Representatives aka "lower house") and the 参議院 {sangiin} (House of Councilors aka "upper house") — of the bicameral 国会 {kokkai} (National Diet aka "Japanese parliament") majority (>50%) vote/agree to just give you citizenship, no strings or conditions attached. Thus, in theory (and I do mean this literally because there's not a single example of it ever happening), all of the requirements for naturalization (including renouncing your other citizenships and not having a criminal record), don't apply.
   
   What do you have to do? Who knows? Jump in front of a madman with a gun in front of the imperial palace and save a member of the royal family, similar to Harrison Ford in "Patriot Games" (he was knighted, but he didn't get citizenship, by the way)? Retrieve and rescue the Japanese passengers from a crashed burning jumbo jet, similar to Dustin Hoffman in "Hero" (he didn't want or get anything either, not even a key to the city, by the way)? †
   
   Invent a way to grow rice without water?
   
   
2. 普通帰化 {futsū kika} (regular naturalization)
   
   This is the path you take if your single or married to a non-Japanese. This means five years straight in Japan, don't be a minor, don't be poor, don't have a criminal record or cheat on your taxes, be willing to get rid of your other citizenships, and don't be an anarchist.
   
   
3. 簡易帰化 {kan'i kika} (simplified naturalization)
   
   For 99%‡ of the people, this is the path you take if you're married to a Japanese citizen for more than one year. The five year residency requirement drops to three years. If you're married for more than three years, then the residency requirement drops to ONE year. This is actually very similar to the residency requirements for those applying for 永住者 {eijūsha} (permanent resident) status who are married to Japanese (three years of marriage and one year of residency), but not the same.
   
   The "simplified" means that one or more of the six requirements for naturalization have been made easier.

† Sorry, my knowledge of old unlikely-hero movies stops at 1992, the year I came to Japan.
‡ I made this percentage up and I have no data to back it up.

Sometimes 簡易帰化 {kan'i kika} (simplified naturalization) is called 特別帰化 {tokubetsu kika} (special naturalization). This has no direct connection to the 特別永住者 {tokubetsu eijūsha} (special permanent resident[s]; a permanent resident with WWⅡ connections to Japan), even though many special permanent residents would probably qualify for "special naturalization."

What Gets You "Simplified"

JAPANESE FAMILY & TEN YEAR-RESIDENT

What Gets Simplified:

• The continuous five years of stay in Japan requirement

Who Qualifies for this Simplification:

1. One who has had a domicile or residence in Japan for three consecutive years or more and who is the child of a person who was a Japanese national (excluding a child by adoption)
2. One who was born in Japan and who has had a domicile or residence in Japan for three consecutive years or more
3. One whose father or mother (excluding father and mother by adoption) was born in Japan; and
4. One who has had a residence in Japan for ten consecutive years sometime in the past, even if they've since broken their consecutive years streak since then.

COMMENTARY:

As this shows, blood relations (jus sanguinis) to Japanese helps with naturalization, but does not make you an automatic shoe-in. And unlike the U.S., where being born there (jus soli) practically guarantees citizenship (and the right to be president), being born in Japan only gives you a two year residency simplification. Fun trivia: contrary to popular belief, U.S. military bases and embassies in Japan actually considered Japanese soil and not U.S. soil. I've known a few military kids who were born on U.S. bases in Japan that would probably be amused to know they might qualify for simplified naturalization after finishing a three year stint in the JET Programme.

FORMER JAPANESE

What Gets Simplified:

• The continuous five years of stay in Japan requirement
• The twenty years or older (an adult in Japan) age requirement
• The secured livelihood (aka "don't be poor and have a job") requirement

Who Qualifies for this Simplification:

1. One who is a child (excluding a child by adoption) of a Japanese national and has a domicile in Japan;
2. One who is a child by adoption of a Japanese national and has had a domicile in Japan continuously for one year or more and was a minor according to the law of its native country at the time of the adoption;
3. One who has lost Japanese nationality (excluding one who got Japanese nationality by naturalization) and has a domicile in Japan; or
4. One who was born in Japan and has had no nationality since the time of birth, and has had a domicile in Japan for three consecutive years or more since then.

COMMENTARY:

I have no idea how many people have ever qualified for condition #4, or why that counts as a "former Japanese." 

Note that unlike many countries in the world (like the U.S.), the Japanese Constitution explicitly guarantees the right to leave Japan and/or renounce your citizenship (Article 22 in Chapter Ⅲ, The Rights and Duties of The People aka the Japanese "Bill of Rights"). 

But if you renounce your naturalized citizenship, that's it. Game over. You can never annul it for all practical purposes, so you'll have to work your way up from visa to PR (optional in Japan) to passport all over again. You changed your mind and don't want to live in your new country and you want to be come back again? Tough. "A woman country scorned," as they say. Go be Canadian* instead.

* A mighty fine country I might add, with some of the best comedians and hockey clubs in the world. Thanks to many years of NHL season tickets, I can sing "O Canada" with confidence. I still can't sing 君が代 {Kimi ga yo} (The Japanese National Anthem). Fortunately, singing the Japanese anthem or even remembering it is not a condition for naturalization.

SPOUSE OF JAPANESE

What Gets Simplified:

• The continuous five years of stay in Japan requirement (to either one or three years; see the "who" below)
• The twenty years or older (an adult in Japan) age requirement

Who Qualifies for this Simplification:

1. One who has had a domicile or residence in Japan as a spouse of a Japanese national for three consecutive years or more;
2. One who has been married to the Japanese national for three years or more and has had a domicile in Japan continuously for one year or more.

COMMENTARY:

Saved the most common scenario for last. This is the section that most everybody (including me) qualifies for if you did the cliché I married a Japanese and started a family here so I love it here and am going to spend the rest of my life here scenario. Qualifying for the dropping of the age requirement means that you somehow legally married a Japanese national as a minor in your home country. I did the math and checked the minimum age for marriage in various states in the U.S., and yes, it turns out that technically speaking, you could marry a Japanese at the age of fourteen in some states, move to Japan at 18 on a spouse or dependent visa, and qualify for simplified naturalization by 19 years of age.

★ For the record, I do not encourage or recommend teenage marriage — to Japanese or anybody else for the matter. Keep reading 漫画 {manga} (Japanese graphic novels) and watching アニメ {anime} (animé), kids, but you need not ask that cutie exchange student from 静岡県 {Shizuoka-ken} (Shizuoka Prefecture) for their hand in marriage when you really just want to go to the prom. Stay in school, keep studying Japanese, and be patient. ☮