PURPOSE: To reduce the fluctuation range of input current corresponding to operating frequency by a method wherein operating frequency range of a compressor is divided into two or more zones, the operating frequency is reduced by a specified value when the input current exceeds the operating value in the zone and increased by a specified value when it drops below the cancellation value.

CONSTITUTION: When a compressor 4 is started, a controller 7 detects the set operating frequency, examines it to decide which zone it is included in, obtains electric current release operating value X1 and a cancellation value X2 of the input current in the zone from a memory 8, and compares the input current Z of an input current detecting circuit 5 with the operating value X1 and the cancellation value X2. When the input current Z exceeds the operating value X1, the operating frequency is reduced by a specified value, and when the input current is between the operating value X1 and the cancellation value X2, operation is continued with the operating frequency remaining unchanged. When the input current Z is below the cancellation value X2, the operating frequency is increased by a specified value. Thereby, the input current is restrained from increasing before it reaches an abnormal current value at which a temperature detecting element is activated, and a compressor 4 is prevented from stopping.