[Answer]
Birthright citizenship is conferred either by jus sanguinis
(citizenship attribution by parentage) whereby a child takes its citizenship
from his parent, or by jus soli (citizenship determined by the place of birth)
by which citizenship is extended to any individual born in the territory of the
state. Jus sanguinis is branched into two main principles – patrilineal jus
sanguinis whereby citizenship is determined by having a father who is a
national of the state, and dual system of jus sanguinis whereby citizenship is
transmitted to a child if either of its parent is a citizen of the state. Japan
adopts the dual system of jus sangiunis. On the other hand, under the laws of
the United States and some south American countries any person born within the
territory is automatically granted citizenship. Some countries have a combination
of jus sanguinis and jus soli policies so that a primarily jus sanguinis
country can bestow citizenship by virtue of place of birth and vice versa for
the sake of convenience.
In your case, since one of the parents is Japanese, your
child acquires Japanese citizenship and depending on the law of your country,
your child acquires citizenship of your country and keeps dual nationality.
Registration of Birth with your city
office should be completed within 14
days of the birth of your child. If your government extends nationality to your
child, you also have to report its birth to your embassy or consulate general
in Japan within a specified timeframe. A Japanese child with dual nationality
is treated as Japanese under Japanese law and a resident certificate will be
created upon registration of the child’s birth.
Your child takes your
Japanese spouse’s surname because he will be entered in the family registry of
the Japanese parent. If you want your child to have your foreign family name,
there are two ways to do so. One is to change your Japanese spouse name to your
foreign surname before giving birth to a baby (in this case your child will be
entered in your spouse’s family registry). And the other way is to apply to the
family court for the change of the child’s family to that of your surname (in
this case your child’s independent family registration will be created.)