A structure and method for fastening a ball seat for a ball valve, a trunnion-type ball valve (100), and a hydrogen station using this valve, in which a stepped part (30) is provided in mid-course of each of inner and outer peripheral surfaces of a resin-made annular seat body (21), with a seat part (31) provided on a tip side and a holding part (32) provided on a rear end side; an attachment groove (45) formed of inner and outer cylinders having a ring-shaped swaging edge part (34) is formed at a tip of a seat retainer (22); the holding part (32) is attached to this attachment groove (45); and the swaging edge part (34) of the inner and outer cylinders is closely contacted and fixed by being swaged to the stepped part (30) to cause the seat part (31) to protrude more than a swaging position.