To hold pressure of compressed air passing an air feed pipe to a constant level regardless of change of the using rate of compressed air in a demand by interposing a continuously variable transmission in a power transmitting passage between an oil cooling type air compressor and a motor for driving the air compressor.

In an air compressing unit, when a compressor 1 is driven by a motor 28, air is intaked into the compressor 1, air is compressed, compressed air is led into an oil separator 6, and compressed air is supplied to a demand after oil in compressed air is separated. In this case, a continuously variable transmission 30 is interposed in a power transmitting passage arranged between the compressor 1 and the motor 28. In the case where pressure of compressed air which is passed in an air feed pipe 31 is fluctuated by change of the using rate of compressed air in the demand, the detected signal of a pressure sensor 32 is applied, the continuously variable transmission 30 is controlled by the controller 33, a change ratio is changed, and thereby, rotational speed of the compressor 1 is regulated, and pressure of compressed air which is passed the air feed pipe 31 is held to a constant level.