[Answer]
You nationality
won’t change automatically with marriage to a Japanese. In order to obtain
Japanese nationality, you ought to apply for naturalization with your legal
affairs bureau. The conditions for naturalization of a spouse of Japanese are
less rigid than those required of other foreigners staying with different
resident status. To be qualified for naturalization, you should have stayed in
Japan for longer than three years or else your should have been married for
longer than 3 years and have been staying in Japan for more than one year,
while normal naturalization requires five years long continuous residency.
Under the Civil Code, if both husband and wife are Japanese,
either of them must change their family name after they get officially married.
But this rule does not necessarily apply to marriage between a Japanese and a
foreigner. Whether or how your family name changes depends
on the requirements in your country’s international law. Namely, ①which country’s law will be used in the event you marry a foreign
national, and ②what the chosen law says about your
family name after marriage. As for the entry to your spouse’s Japanese family
registration, in principle, your family name will be used as is. But if you
take your spouse’s surname after your marriage, you can change the entry
accordingly if you apply for “change to your spouse’s family name” with your
city office. To apply for this change, you need to submit official documents to
that effect. In a family registration, a name of a foreigner is usually written
in katakana but if you take your Japanese spouse’s surname, your surname will
also be written in Kanji Chinese characters.