PURPOSE: To reduce the power consumption of a compressor by controlling the voltage of a driving means and the position of a switching valve which changes over the connection of the discharge side of a compressing means to an auxiliary cooler or a by-pass piping.

CONSTITUTION: A detecting element 10 to detect the oil temperature of lubricating oil 2 is provided at the bottom of an elclosure 3, the output of said detecting element being inputted to a microcomputer 11 to change over a switching valve 7 to connect the discharge side of a compressing means 5 to an auxiliary cooler 8 when the temperature of the lubricating oil 2 is high and to a by-pass piping 8 when the said temperature is low while also controlling the voltage of driving means 4 according to the temperature of the lubricating oil thereby maintaining the temperature of the lubricating oil constantly at the temperature where its viscosity is the lowest to reduce the input and reducing the voltage by the amount of input reduction to reduce the power consumption.