An air supply circuit is provided with: a compressor which selectively performs loaded operation and idle operation; a desiccant which removes water content from compressed air sent from the compressor; an instructing unit which outputs to the compressor a first signal indicating that compressed air is not required; a control valve which is connected to a location between the compressor and the desiccant and which is selectively switched between a communicating position and a discharging position; and a generating circuit which outputs a second signal generated on the basis of the first signalto the control valve. When the instructing unit has output the first signal, the compressor performs idle operation in accordance with the first signal and the control valve switches to the discharging position in accordance with the second signal. When the instructing unit has stopped the first signal, the compressor performs loaded operation in accordance with the stopping of the first signal and the control valve switches to the communicating position with a delay of a prescribed length of time after the first signal has stopped.