Wikipedia
A (literally "adopted son-in-law") is an adult man who is adopted into a Japanese family as a daughter's husband, and who takes the family's surname.
Generally in Japan, a woman takes her husband's name and is adopted into his family. When a family, especially one with a well established business, has no male heir but has an unwed daughter of a suitable age, she will marry the mukoyōshi, a man chosen especially for his ability to run the family business. If there is no daughter, the candidate can take a bride from outside his adopted family (Fufu-yōshi). This is done to preserve the business and name of the family when there is no suitable male heir, since traditionally business are inherited by the oldest male heir. Mukoyōshi is also practiced if there is no capable male heir to run the family business.
This is a centuries-old tradition and is still widely practiced today. Many Japanese companies with household names such as Suzuki, Kikkoman, and Toyota have all adopted this practice.
This adult adoption may take place in marriages where the woman's family is of a higher socioeconomic rank than the man's family, where the woman has no brothers to be the heir to the family name, when the man has been disowned by his own family, or when the man's natural family comes from a notorious or shameful background and he thus prefers to hide his identity.