If you wish to marry in Japan, you will do so according to Japanese law. Marriage in Japan consists of a civil marriage registration by the couple at a Japanese municipal government office.

Only this civil registration constitutes a legal marriage in Japan. Ceremonies performed by religious or fraternal bodies in Japan, while perhaps more meaningful for the couple, are not legal marriages. Consular officers cannot perform marriages.

Article 731 to 737 of the Japanese Civil Code stipulates the following requirements:

* The male partner must be 18 years of age or older and the female partner must be 16 years of age or older.
* A woman cannot get married within six months of the dissolution of her previous marriage. According to Japanese law, this is to avoid confusion as to the identification of a child's father if a birth occurs close in time to the end of the marriage.
* Most people related by blood, by adoption or through other marriages cannot get married in Japan.
* A person who is under 20 years of age cannot get married in Japan without a parent's approval.

Japanese law requires all foreigners who marry in Japan to first prepare a sworn Affidavit of Competency to Marry (Konin Yoken Gubi Shomeisho 婚姻要件具備証明書), affirming they are legally free to marry, from their own country's embassy or consulate in Japan.
SOURCE: Here