Changes to Koseki

Hello.
I have question about family registration (Koseki) change.
When i get married ten years a go in Japan, my wife (Japanese national) registered family on her name. There was no need as we lived in europa at thet time. Now we moved back to Japan and i can not register kids on my name anymore, because Koseki is on my wife's name. We went to court, and they said i have to write letter to with reason, why we need to change family name. Consultant at court added, that my story is not good enough to make court change my Koseki.
Does anyone had similar problem?
Can someone suggest me lawyer who know how to write this letter?
Thank you.

Please keep in mind that "koseki" is something only Japanese nationals have.

So when you got married, her marriage to you went in the "remarks" column of HER koseki as "gotten married to Mr. So-and-so on such-and-such date according to the Japanese law." If she doesn't take the specific procedure to change her family name to yours, upon marriage, her family name remains unchanged. (And her child(ren) would go into her "koseki," meaning, in her family name.)

And it takes a court proceedings if she wants to change her family name after a certain period of time, I believe it was after three or six months after the marriage, "if" she has pressing reasons why keeping the current name is unreasonably inconvenient for her life.

Now considering that you are in Japan, the family court would hardly see a reason why she would find it "unreasonably inconvenient" to keep her own family name...

In our case, (myself = Japanese and husband = non-Japanese) we got married in another country where we used to live in, kept our family names as they were, so my name remains unchanged. Only on my passport I have my husband's family name in parenthesis after mine, so that we can more easily show we are a married couple traveling together. No children here.)

Just to add... the name on the Koseki is the official name and doesn`t need to be used for every day things - like school, friends, eventual work, etc. And over time, if another name is used, it is easier to get a court to permit an official name change on the basis of `commonly known and used name causing an inconvienience if the application is not approved`. That of course would be some time in the distant future for a child, if that child indeed wanted to use another (your) surname.

I dont know the kind of reason they need from you ...
she want her family to be in one family name is a very good reason .

i married my wife since 2011 and she didn`t change her name to my until 2016 december i went to register my new born baby with my wife mother at city office . there i found out that my baby can not be register under my name unless my wife change her name to my family name.

so last year 2017 she applied to change name which they approved.

the only reason is that she want her family to be in thesame family name .
all my wife did was
1, get the application forms fill it up at home
2, submit to family cout with supporting documents.