His father was Kushima Masashige, a vassal of the Imagawa clan.

[1] In 1545, during the Siege of Kawagoe Castle, despite an overwhelming attacking force, numbering around 85,000, the 3,000 men Kawagoe Castle's garrison under Hōjō Tsunashige, held off the siege until the Hojo Ujiyasu relief force arrived.

[1] In 1551, he fought in Siege of Hirai castle against Uesugi Norimasa.

In 1571, Fukazawa Castle that Tsunashige defended was surrounded by Takeda Shingen's large army.

[3] After the Siege of Fukazawa Castle, Tsunashige retired from public life, and he died in 1587.