Whose family register do you go onto?

If you are married to a Japanese when you decide to take Japanese citizenship, there are (at least) two decisions you will need to make not by yourself, but with your spouse as well:

1. What to do about the family name? When you are a foreigner married to a Japanese, the two can have 別姓 {bessei} (separate last names) because the foreigner is not part of a Japanese family register, which groups all family members under the same family name. Once you naturalize, though, you have to unify under the same name for the purpose of a unified single Japanese family register. This means one of three options:
1. Does your spouse take your foreign name, translated/transliterated into Japanese script: 漢字 {kanji} (Japanese sinograms) using 当て字 {ateji} (phonetically/meaning approximated characters), or 仮名 {kana} (Japanese syllabet)?
2. Do you take your spouse's name?
3. Do you pick an entirely separate new family name, and both of you go with this name? This could be a combination of your two surnames together, both written in Japanese script. Keep in mind that legal family names cannot have dashes, spaces, or other forms of punctuation like English names that do this (ex. Rodriguez-Samson). Also keep in mind that while there is no legal length limitations for Japanese names on a 戸籍 {koseki} (family register), there is a pragmatic length restriction of about 7 Japanese script characters for an entire name before it becomes impractical for daily use in Japanese society.
2. What to do about the family register?
1. Do you create your own family register, and your spouse and children join you on it?
2. Do you join your spouse on their family register?

Traditionally speaking, a woman who marries a man usually joins the man on their family register. However, this is not always the case. There are cases when a man joins the woman's individual (or even the woman's family's) register. Some of these factors may include one or more of the following conditions:

• If the woman comes from a family of particularly high status or power compared to the male
• If the woman is the last in the lineage line for her family and/or name
• If the man wishes to distance himself from his own family lineage (for example, scandal or shame)

The act of a man joining the woman and/or her family's register is called 婿入り {mukoiri}. It is very uncommon, but not rare. In fact, 婿入り {mukoiri} happens more often in Japan than men using the woman's family name in the United States.

Note that even if you choose to use your spouse's name, this does not necessarily mean you are joining them on their family register. It can just be a "coincidence" that the two of your had the same name. For example, it's possible for two Japanese to marry one another and already have the same name; it does not mean that they are remotely related.

For example, I chose to use my wife's last name for pragmatic reasons for life in Japan. However, I created my own 戸籍 {koseki} (family register) but used the exact same family name as my wife. Thus, she did not have to change her name, even though she (and my child under her) left/dissolved her register to transfer all the information onto mine.

Nothing about the family register is permanent, by the way. Once your children become legal adults, they can opt to split off of your family's family register and create their own family register, with them at the top. Likewise, should you and your spouse divorce, the family registers are again split into two different registers — even if you don't change your family name. There will be records on all the relevant registers regarding the chain of 除籍 {joseki} (removal from a register) and 転籍 {tenseki} (transfer to another register) regarding any transfers and removals or other changes, so one could follow the chain of transfers and changes, if they had the ability to access all the documents, to determine older compositions or names.

When the naturalization process is near completion and they are verifying the details for the family register, both your case officer and the 市役所／区役所 {shiyakusho/kuyakusho} (city hall / ward office) officials will discuss how your family wishes to construct the family register.