PURPOSE: To avoid the insufficiency of oil at the compressing part of a compressor, even when air temp. drops largely.

CONSTITUTION: When the temp. of cooling air measured by a temp. sensor 52 falls to a prescribed temp. (e.g. 0°C) or below, the frequency of an electric power supplied to a fan motor 16M is decreased to reduce the revolving speed of cooling fans 16A and 16B. Therefore, when air temp. falls largely, the cooling action of an oil heat exchanger 12A is automatically lowered and the temp. of the oil flowing through a side passage pipe 36 will not drop below a prescribed temp. For this reason, even when air temp. falls largely in the winter in a cold district, the viscosity of the oil is increased and the oil flow amount does not become insufficient, an invariably sufficient amount of the oil is supplied to a compressor 11 and the operation is performed in a normal condition throughout the year. Accordingly, the cooling operation of a very low temp. cooler 23 is performed always in a normal condition.