In a pressure vessel which is formed by blow molding a plastic liner with a mouth member fitted on a blow pin, a rotational torque is prevented from being applied between the mouth member and the blow pin when removing the blow pin. A parison is placed around the mouth member which is fitted on the blow pin, and is blow molded with a die assembly placed around the parison so that a vessel shaped plastic liner is formed. An engagement structure which is provided between the mouth member of the pressure vessel and the blow pin includes a ball moveably received in a radial through hole of the peripheral wall of the blow pin, and a release groove formed on an inner circumferential surface of the mouth member and having a prescribed depth. When the blow pin is introduced into the mouth member until the ball and the release groove oppose each other, the ball moves radially outward and becomes partly received by the release groove to engage the mouth member with the blow pin.