To enable a specific high-precision stroke adjustment by rotatably mounting an adjustment ring on a cylinder head axis to be rotatably coupled with a stop member, enabling the displacement of the stop member in an axial direction, and providing a graduation for reading a stroke on the adjustment ring.

The adjustment of a pin stroke is carried out by pulling out a graduation member 60 to the left side until a pin 68 abuts on a step 66, and turning an adjustment ring 44. The rotation is transmitted to a slider 38 through a bolt 42. A stop member, namely a bolt screw portion 34, shaft 36, the slider 38, and a stop head 40 undergo displacement in an axial direction to a piston 16 by a degree defined with a screw pitch to adjust a stroke, namely the distance between the stop head 40 and a shoulder portion 52. The stroke is read from rough graduations on the graduation member 60 and fine graduations 56 in a circumferential direction. If the graduation member 60 is pressed in, the stroke is maintained. Therefore, a high-precision stroke adjustment of 1/10 mm and more is enabled.