PURPOSE: To prevent shortage of oil in a compressing part of a compressor even when the temperature of outside air falls in a large degree.

CONSTITUTION: At the end part of a hood 71 which leads outside air sent by a cooling fan 16 to an oil heat exchanger 12A, a screen 73 which is made of a shape memory alloy taking two different shapes according to temperatures and which becomes a flat plate at a temperature being lower than a prescribed temperature and takes a curled-up shape at a temperature higher than that temperature is provided for adjusting the area of the opening of a flow passage 72. Besides, electric heaters 42 and 43 which are electrified to heat when the temperature of oil measured by a temperature sensor 52 lowers to the prescribed temperature are provided on an oil return route 36. Ehen when an outside air temperature falls in a large degree in the winter in a cold district or the like, therefore, no shortage of the flow rate of the oil due to an increase in the viscosity thereof takes place, a compressor 11 is always supplied with the oil in a sufficient quantity and operated in a normal state through a year, and consequently a cooling operation in normal conditions is always executed in a very low temperature cooler 23.