If your child is born in Japan, get the Birth Certificate from the hospital and submit it at the Registers Section of the City Hall within 14 days of the birth (inc. the day of birth). *When a birth notice is submitted the child will be listed on the resident record as a “person who may continue to stay transitionally in Japan by birth” which is valid for 60 days.
In addition you have to go through the following procedures. Also refer to B-4-1 Pregnancy & Childbirth of Guidebook Soka.