Mishima was born in the Yotsuya district of Tokyo (now part of Shinjuku). His father was Azusa Hiraoka, a government official, and his mother, Shizue, was the daughter of the 5th principal of the Kaisei Academy. Shizue's father, Kenzō Hashi, was a scholar of Chinese classics, and the Hashi family had served the Maeda clan for generations in Kaga Domain. Mishima's paternal grandparents were Sadatarō Hiraoka and Natsuko (family register name: Natsu) Hiraoka. He had a younger sister, Mitsuko, who died of typhus in 1945 at the age of 17, and a younger brother, Chiyuki.