What is the equivalent of a birth certificate for someone born in South Korea?

We're trying to fill out immigration papers and they're asking for a birth certificate equivalent, but there doesn't seem to be a strong equivalent for it in South Korea.What is the equivalent of a birth certificate for someone born in South Korea?
An offical letter indicating that the individual's name appears on the Korean Family Census Register (preferably with a copy of the appropriate section of the registry), and a valid Republic of Korea passport.



These are the official documents necessary to prove citizenship.