To guarantee sure holding of a wheel material at the time of spinning process.

When a tail stock cylinder expands, a cam member 32 moves leftward together with a movable sleeve member 26, and first and third cam surfaces 32b and 32d slide on a first frictional contact surface 18b on the outer surface of a rocking arm 18 and a third frictional contact surface 18d on the inner surface, to swing the rocking arm 18. Following the swinging, a circumferential groove 18a of the rocking arm 18 and a circumferential groove 14b of a rocking shaft 14 bite each other. When the tail stock cylinder contacts, the movable sleeve member 26 and the cam member 32 are moved rightward by the energizing force of a return spring 30. A second cam surface 32c of the cam member 32 slides on a second frictional contact surface 18c of the rocking arm 18 to swing the rocking arm 18. Following this, the circumferential groove 18a of the rocking arm 18 departs from the circumferential groove 14b of the rocking shaft 14.