To prevent burning damage of a motor beforehand by arranging a plurality of air compressors, driving each air compressor by a single motor through an electromagnetic clutch and a belt respectively, and also opening the electromagnetic clutch when the current value of the motor exceeds a setting value.

When two air compressors 1 (1a, 1b) are driven by one motor 36 through belts 12, 12b and an electromagnetic clutch 11, air compressed by each compressor 1 is absorbed and compressed into each compressor 1 through air filters 2a, 2b, and compressed air is stored in an air tank 6 through discharging pipes 5a, 5b. In this case, The capacity of the motor 36 is set when a maximum flow rate is operated with a rated pressure by two compressors 1. At the time of small flow rate, operation is carried out by only one compressor 1, current of the motor 36 is increased when setting pressure is raised to the maximum pressure level, the electromagnetic clutch 11 is opened when the setting of the excessive current relay exceeds, and then operation of the compressor 1 is stopped.