PURPOSE: To reduce the driving torque of a compressor, by providing pressure reservoirs on both sliding surfaces of a twin heads type piston and a slider so that the reservoirs are separate from each other and by separately connecting the pressure reservoirs to the compression chambers of cylinders through the communication holes of the piston heads.

CONSTITUTION: When a driving shaft 3 performs one rotation, pistons 8, 8 reciprocate in cylinders 16, 17 to suck and discharge coolant gas into and out of the compression chambers of the cylinders through valves. At that time, the collant gas of low pressure flows into the opening 24 of a body through an inlet port 23, and then flows into a sucked gas chamber 27 in a valve cover through a sucked gas passage 25 and both communication holes 26 and the collant gas of high pressure released into a discharge gas chamber 30 in the valve cover is sent out an outlet port 33 through a slender hole 31 and a discharged gas passage 32.