An annular mating portion 42 extends from a fastening surface 40a of a fastening nut 40. The mating portion 42 has an inner diameter set to be substantially equal to an outer diameter of the portion of a piston 25 in which an accommodating groove 36 is formed. In a state in which the mating portion 42 is inserted into the accommodating groove 36, and an inner circumferential surface 42a of the mating portion 42 slides in contact with a circumferential wall 36a of the accommodating groove 36, the fastening nut 40 is mated with a threaded portion 39, and a lip seal 37 is held between the mating portion 42 and the accommodating groove 36. This fixes the lip seal 37 to the piston 25.