To easily manufacture a metal mold by making upper side opening windows of a piston and a valve case in the same shape except for opening parts.

Opening areas are sequentially reduced by sequentially increasing radi Rd, Re, Rf, Rg reaching the inner peripheral side of the upper side opening window from a shape center by separately providing an upper side boss part 105L or 115L formed on the inner peripheral side, a radial window frame 105M or 115M extending in the radial direction on the outer peripheral side as well as connected to an upper end surface of the upper side boss part and a plural number of fan type upper side opening windows 105D, 105E, 105F, 105G or 115D, 115E, 115F, 115G surrounded by a circular arc window frame 105N or 115N continued to the radial window frame with equal intervals along the circumferential direction on the upper surface side of the piston 105 or the valve case 115 against which a check valve or a suction valve faces, making each of the upper side opening windows the same in an opening angle θa between the radial window frames and in a radius reaching the arc window frame from the shaft center and sequentially increasing the radi Rd, Re, Rf, Rg reaching the inner peripheral side of the upper side opening window from the shaft center.