An annular mating portion extends from a fastening surface of a fastening nut. The mating portion has an inner diameter set to be substantially equal to an outer diameter of the portion of a piston in which an accommodating groove is formed. In a state in which the mating portion is inserted into the accommodating groove, and an inner circumferential surface of the mating portion slides in contact with a circumferential wall of the accommodating groove, the fastening nut is mated with a threaded portion, and a lip seal is held between the mating portion and the accommodating groove. This fixes the lip seal to the piston.