PURPOSE: To avoid rated operation of an air compressor as much as possible and to permit it to mostly operate at a low speed, by providing desired first and second reference values between values set at the upper and lower limits of air pressure in a source air storage.

CONSTITUTION: When pressure P reaches a value set at the upper limit P4, a first comparison unit 12 of a pressure regulator 4 makes an output E1 to L and when it is less than a first reference value P2, it makes an output E12 to L. When the pressure P reaches a second reference value P3, a second comparison unit 14 makes an output E14 to L and when it is less than a value set at the lower limit P1, it makes the output E14 to H. A timer 15 makes an output E15 to H when an input E14 is H and a prescribed period of time has passed and it makes the output E15 to L at once when an input E14 is made to L. A starting unit 5 gives priority to an H signal of E16 for rated operation when an output E13 and the H signal of the E16 are applied. With a property as described above, an air compressor 1 is operated as shown in the drawing, whereby it is started an d stopped less frequently and continues operation at a low speed.