Collecting Koseki (Family register) documents from Japanese local municipalities is essential for Japanese Genealogy.
It's not so difficult to collect Koseki documents if you have a Japanese nationality with residential registration in Japan.
But especially for Nikkei people, the descendants of immigrants from Japan, it becomes much harder to collect Koseki since Japanese law protect personal information very strictly.
If you wish to gain your ancestors' Koseki, you're required to submit to Japanese local municipalities any official proofs to show the linkage between you and your ancestors such as your and your parents' birth certificates, your parents' marriage certificate, and your grand parents' death certificate.....if it's in such cases.
In addition to Koseki, there are many ways to pursue Japanese Genealogy based on Real Property registrations, Military service record, Ancient manuscripts, Buddhist lists of a local temple, and so on.
If you wish to know more about Japanese Genealogy, just drop me line.