PURPOSE: To prevent a compressor from unnecessarily being reduced in the capacity and from hunting by a method wherein the sampling interval is shortened depending not only upon the absolute value of the delivery pipe temperature of the compressor but also upon the judgment of trend of rise or fall in the temperature.

CONSTITUTION: The delivery pipe temperature TD of a compressor 1 is detected by a delivery pipe temperature detector Th7, and compared with a temperature previously detected to decide the trend of rise or fall in the delivery pipe temperature TD. When it is judged that the delivery pipe temperature TD tends to rise, the operating capacity of the compressor 1 is reduced by a controller 61 at regular time intervals. On the other hand, it is judged that it is other than rising trend, the operating capacity of the compressor 1 is maintained and controlled according to the absolute value of the delivery pipe temperature TD and the trend of rise or fall in the temperature. Thereby, the compressor 1 is prevented from unnecessarily being reduced in the capacity and from hunting, causing an extension of continuous operating range.