under the high load but does not reach the maximum displacement thus the following equation pd lt pc pp is always satisfied when the spool 44 is caused to open the refrigerant gas in the discharge chamber 25 is discharged from the opening 35 into the crank case 5 through the passage 47 of the spool 44 the valve chamber 43 and the communication passage 50 respectively as a result the internal pressure in the crank case 5 is caused to increase the rapid flow of the refrigerant gas from the discharge chamber 25 to the crank case 5 and the rapid internal pressure increase in the crank case 5 and the rapid internal pressure decrease in the discharge chamber 25 are prevented by the throttle 48 in the passage 47 the compression displacement is smoothly varied to obtain the steady operation of the compressor while the compressor is operated at the maximum displacement the wobble plate 18 is caused to rock forward and backward in relation to the rotary movement of the drive shaft 6 the refrigerant gas is compressed by the reciprocating movement of the piston 9 within the cylinder bore 8 the pressure pc in the crank case 5 is controlled by the automatic control valve 51 the inclination angle of the wobble plate 18 is varied according to the difference between the pressure pc in the crank case 5 and the pressure ps in the suction chamber 24 in order to control the compression displacement the spool 44 is caused to open within the valve chamber 43 accommodated in the passage 42 the refrigerant gas is caused to rapidly flow into the crank case 5 from the discharge chamber 25 so as to decrease the inclination angle of the wobble plate 18.