capital, Gumma ken (prefecture), Honshu, Japan, on the Kantō Plain. An old castle town, in the Muromachi period (1338–1573) it was called Umayabashi. It was the seat of the Matsudaira family during the Tokugawa period (1603–1867). The city grew rapidly after World War II as an industrial and communications centre. It continues to produce silk yarn. Pop. (2005) 318,584.