PURPOSE: To cause a capability of a compressor to be continuously varied in stepless manner according to a freezing load by a method wherein a working chamber communicates with a suction side of a compressor through a bypassing pipe and a control valve of which degree of opening is controlled according to a pressure or a temperature of refrigerant at a suction side of the compressor is installed at this bypassing pipe.

CONSTITUTION: A bypassing pipe 35 for connecting a compressor 30 with a suction pipe 34 is provided with a control valve 36. This control valve 36 is controlled for its degree of opening under an instruction from a controller 37. To this suction side pipe 34 is attached a sensor 38 for use in sensing either a pressure or a temperature of refrigerant flowing in the suction side pipe 34. A signal from this sensor 38 is inputted to a comparing means 39 of a controller 37, where it is compared with a set value set in a setting means 40 and then a difference between both values is calculated. This difference is inputted to means 41 for determining 42 and then a degree of opening corresponding to the difference is determined. The memory means 42 stores a predetermined degree of opening in correspondence with the difference in the table. This degree of opening is outputted to the control valve 36 through an outputting means 43 and the control valve 36 has this decided degree of opening.