The bottom surface of a seal groove provided in a cylinder bore has a first slant portion and a second slant portion. A transition point between the first slant portion and the second slant portion is located closer to the opening of the cylinder bore than the center in the cylinder bore axial direction of the seal groove. A gap is formed between the outer peripheral surface of a piston seal member and the bottom surface of the seal groove, and the surface pressure applied to the bottom surface of the seal groove from the piston seal member is higher at the cylinder-bottom-side end of the piston seal member than at the cylinder-opening-side end thereof. Thus, it is possible to suppress the movement of the piston toward the cylinder bore bottom due to thermal expansion of the piston seal member.