First of all, death records are not searchable in Japan due to the strict protection policy of personal information. It is far from an easy task to complete the death verification with an official document. It requires arduous efforts to complete this mission.
Passport number mostly does not make any sense in terms of private investigation due to the authoritative government policy. Immigration Bureau does not disclose its information to the private sector even with a legal reason.
And since the Japanese government has not adopted ID number system for its citizens, we need to know the residence address or the Honseki (legal domicile) on Koseki family registry records of the person in order to obtain vital records (Koseki family registry records).
There is a chance we can locate the residence address in Japan if he is also a company representative in Japan.
We are to obtain company registry records of the person, find his residence address, and obtain his Koseki family registry records with the help of our legal adviser (only attorneys have a professional claim right for Koseki family registry records).
Our fee is US$1,500.00 in total to obtain the Koseki records. Turnaround time is about 2 weeks.