How can I get my koseki in japan?

I was born in Japan and a japanese citizen.I am now 21 years old.I have my expired passport I want to renew it but the problem is i dont have the koseki.We leaved japan when I was 3 or 4 years old because my parents got separated that was year 2000. since then, we did'nt came back.my mother is a Filipina and my father is a japanese.
unfortunately my mother died last 2010.i have no communication with my father i dont even know where to find him to ask his help get my koseki.I was not even send to school because i have no birth certificate.I hope somebody could help me on what to do.

Many word/city/town office accept an oversea request of Koseki-tohon by post.
In that case, you remit comission by Internaitonal Postal Money Order.
So, try to contact the word/city/town office, that your koseki is registerd.
I think you had better show your expired passposrt to them, to convince them that you are real Japanese, uopn inquiry and application.