A structure and method for fastening a ball seat for a ball valve, a trunnion ball valve, and a hydrogen station using the ball valve, in which a stepped part is positioned medially in each of inner and outer peripheral surfaces of a resin-made annular seat body, with a seat part on a tip side and a holding part on a rear end side; an attachment groove defined by inner and outer cylinders having a ring-shaped swaging edge part at a tip of a seat retainer; the holding part being attached to the attachment groove; and the swaging edge part of the inner and outer cylinders being in contact with and fixed by being swaged to the stepped part so as to cause the seat part to protrude farther than a swaging position.