Many of the Kushima family including his father Masashige killed by Toratane HARA from the Takeda clan in the Battle of Iidagawara in 1521, then Tsunashige escaped to Odawara accompanied by his vassals to be protected by Ujitsuna HOJO, and began to serve him as kinju (attendant).
大永元年（1521年）、飯田河原の戦いで父・正成ら一族の多くが武田方の原虎胤に討ち取られ、家臣に伴われて小田原に落ち延びて北条氏綱の保護を受け、近習として仕えた。