To reduce the power for driving a compressor when the coolant circulating quantity is reduced by providing a bypass means for circulating a fluid from a discharge port to a compression chamber to reduce the discharge fluid quantity, which has an intake port for sucking the fluid to a compression chamber and a discharge port for discharging the fluid compressed by the compression chamber.

In a reciprocating type compressor 10 used for heat cycle, a piston 12 fitted into a compressor body 11 is reciprocated through a connection rod 13 by the rotation of a crank 14 to discharge a coolant sucked through an inlet port 11a provided on the upper wall of the compressor body 11 from a discharge port 11b after the pressure is raised. In this case, a bypass circuit 20 for reducing the coolant circulating quantity is provided extending over a coolant outlet piping 17 and the side wall of the compressor body 11 or the side wall of a compression chamber 15. A valve 22 interposed within a piping 21 forming the main body of the circuit 20 is automatically opened and closed by the output signal of a piston position detecting sensor, so that the driving force is reduced when the coolant circulating quantity is reduced.