For many of you, Koseki, family register is very uncommon. 

As long as I know, besides Japan, only China and Taiwan have same kind of system.  Korea used to have it but stop it in 2008.

We, Japanese as well as Chinese, traditionally put great importance on parents and family.  Consequently, we see a family as a unit to live. In old days, they thought family is more important than each individual....

That's why we still have Koseki, family register which is defferent from an individual register sistem such as security code in the USA.
In Japanese, getting married is, in other words, getting into one's Koseki, Nyuseki.  When we marry, we make a new Koseki, leaving our parents' Koseki, traditionally with the surname of a husband.

If you have experience getting married Japanese, you submitted your spouse's Koseki with youe name to Immigration in order to get a spouse visa. It is one of evidences for you to get legaly married.

So maybe you thought you got into your spouse's Koseki.  But actually, no you didn't.   Foreign spouse's name was just added as one of life-time facts.It is like "got married XXXXX".   Please don't think it is a kind of disclimination.   It is simply because Koseki is only for Japanese citizens.

Under Japanese Civil Code, in Koseki, married Japanese couple should have the same surname, either husband's name or wife's name.　　If a child born, he/she has the same surname, too.  For one Koseki, one surname.  Recently this issue has become very hot in Japanese society because many of us want to keep their maiden name even after marriage.  Traditionaly, most couple choose a husband' name.  So many Japanese women think it is against our constitution.

However, if a Japanese gets married with a foreigner, they can keep their own surnames automatically.  Only if he/she wants to change his/her surname,he/she can do it on his/her request.  It is because you, a foreigner don't get into koseki.

What happened to a husband or wife's surname when Japanese couple get divorced?   How about child's one?

Actually, we could choose whether keep a current surname or get back a maiden name.   Children can also choose it.

The same thing happen to your foreign spouse's surname.  If you chose to change your name to your Japanese spouse's surname when got married and want to get your surname back after getting divorce, you report your divorce to your Embassy or Consulate and ask changing your name.

However, for those from a country like the Philippines who do not admit general divorce, you cannot get your name back unless you go through annulment or court recognition procedure.

By the way, I had a good time with my Filippina client, who is a Japanese generation.  

She came to my former office almost 2 years half ago and asked us updating his grand father's KOSEKI and put his father's name on it as a Japanese.  Because of the last world war, he happened to be a stateless. 

After 2 years half, with a lot of effort, we suceeded to update his grand father's KOSEKI and create a new KOSEKI for her father. Unfortunately, it is too late for him because he passed away a year ago. Yesterday, we received his new KOSEKI and shared a joyful time together.

When I see a happy face like hers, I feel my job is not bad.