PURPOSE: To embody the automation by switching operation by only a switching command from an operator, in the case where operation is switched in a compressor having capacity according to an using quantity by change of the using quantity from a common line, in the source air compressor of an air separating device.

CONSTITUTION: In the case where operation is switched from a compressor 1 into a compressor 2 in a plurality of compressors, the compressor 2 is started while closing a regulating valve 2. The setting point of an air quantity signal 41 which is necessary in an air separation device 4 is switched from the compressor 1 into a flow meter 3. The signal of the air quantity at this time is set to the compressors 1, 2 as a setting value. The flow meter 3 is PID (proportional plus integral plus derivative) controlled by the deviation between the setting value from the air separation device 4 and an actually measured value, and the regulating valve 2 is opened, and then the regulating valve 1 is closed so as to set the air quantity in the setting value. In the case where only compressor 1 is operated normally, the signal of the air quantity which is necessary in the air separation device 4 is transferred into the compressor 1 so as to control the amount of air supplied by a compressor main body.