★Acquisition of Citizenship*
Specifically there are two systems in which nationality is conferred

1) citizenship by descent (血統主義＝ketto shugi), which is based on one or both parent's nationality or

2) citizenship by country of birth(生地主義＝seichi shugi) which the place of birth is the deciding factor in a child's nationality and not the parent's nationality per say.

Citizenship in Japan (and France for example) is based on the former (1), whereas the United States, Canada and Brazil base citizenship on the country of birth (2). Exceptions occur in the case that a child is born abroad to U.S., Canadian or Brazilian parents. In this situation, the child's nationality is conferred by descent depending on the parent's residency status in their home country. For example, a child born in the United States to Japanese parents has both U.S. and Japanese nationality until age 20; whereas a child born in Japan to two U.S. citizens is a citizen of the United States only.

Up until 1984 Japan was a patriarchal society meaning that nationality could only be conferred through a Japanese father, not a Japanese mother. Per amendments made to the Nationality Law in 1984, nationality is now based on the child's parents' citizenship; therefore if one parent (male or female) or both of the child's parents are Japanese then the child automatically has Japanese nationality at the time of birth.

★A Child Born Between a Japanese and a Foreign National
For children born between a Japanese and a foreigner, nationality will be decided by the laws of Japan, the foreign country, and the country where a child is born, so it is possible for a child to have dual or even triple-nationality.

 - Acquisition of nationality by an Acknowledged Child
Article 3 of the Nationality Law was changed (and simplified) on January 1, 2008.

"A child (excluding a child who was once a Japanese national) under twenty years of age whose father or mother has acknowledged paternity or maternity respectively , may acquire Japanese nationality through a notification to the Minister of Justice, if the father or mother who made the acknowledgement was a Japanese national at the time of the child's birth, and such father or mother is presently a Japanese national or was a Japanese national at the time of his or her death."

 - the original Japanese version of the law can be found here.

★Acquiring Citizenship through Naturalization
In the case one's biological parents do not marry it is possible to acquire Japanese citizenship through naturalization. If a biological father recognizes the child after birth, the regulations are eased to allow the child of a Japanese citizen to be naturalized. As such, a child can apply for nationalization "as a child of a Japanese". In the case a father refuses to legitimize a child the case can be taken to court and with the decision of a judicial panel the biological father can be forced to legitimize the child.

★Family Registers
Like China and Korea, Japan records familial information in what is called "koseki"（戸籍 = family register). All information regarding, birth, marriage, divorce, residence, and death are recorded on this registry and kept at one's city, ward, or local government office. Only Japanese have family registers or can be legally registered in them. A foreign spouse of a Japanese is noted as such in the spouse's family registry (in the section called mibun-jiko-ran＝身分事項欄).

A child born to a Japanese (father or mother) and a foreigner (mother or father) will be listed in their Japanese parent's family registry.

★Notes:

"Nationality" and "citizenship" are used throughout the article, however the concept of citizenship as used here currently does not exist as a legal term in Japanese. In Japan the concept of citizenship is correctly and only referred to as nationality (国籍＝kokuseki). Please be aware that the term citizenship is used to refer to nationality in this article.