An air supply system, comprising: a compressor including a load operation mode, in which the compressor feeds compressed air, and an idling operation mode, in which the compressor does not feed compressed air; a desiccant configured to remove moisture from the compressed air fed by the compressor; a connection passage that connects the compressor and the desiccant, wherein the connection passage permits the compressed air to flow; a first electromagnetic valve configured to selectively switch the compressor between the load operation mode and the idling operation mode, wherein the compressor is configured to be switched to the idling operation mode when the first electromagnetic valve is activated, and the compressor is configured to be switched to the load operation mode when the first electromagnetic valve is deactivated; a drain discharge valve that is connected to a branch passage branched from the connection passage, wherein the drain discharge valve is configured to open the branch passage when a second electromagnetic valve is activated, and the drain discharge valve is configured to close the branch passage when the second electromagnetic valve is deactivated; and a controller configured to switch activation and deactivation of the first electromagnetic valve and activation and deactivation of the second electromagnetic valve, characterized in that the air supply system further comprises: a downstream check valve that permits air to flow from the desiccant toward a side opposite the compressor; and a regeneration control valve arranged in a passage that is connected in parallel to a passage in which the downstream check valve is arranged, wherein the controller is configured to open and close the regeneration control valve. a compressor including a load operation mode, in which the compressor feeds compressed air, and an idling operation mode, in which the compressor does not feed compressed air; a desiccant configured to remove moisture from the compressed air fed by the compressor; a connection passage that connects the compressor and the desiccant, wherein the connection passage permits the compressed air to flow; a first electromagnetic valve configured to selectively switch the compressor between the load operation mode and the idling operation mode, wherein the compressor is configured to be switched to the idling operation mode when the first electromagnetic valve is activated, and the compressor is configured to be switched to the load operation mode when the first electromagnetic valve is deactivated; a drain discharge valve that is connected to a branch passage branched from the connection passage, wherein the drain discharge valve is configured to open the branch passage when a second electromagnetic valve is activated, and the drain discharge valve is configured to close the branch passage when the second electromagnetic valve is deactivated; and a controller configured to switch activation and deactivation of the first electromagnetic valve and activation and deactivation of the second electromagnetic valve, a downstream check valve that permits air to flow from the desiccant toward a side opposite the compressor; and a regeneration control valve arranged in a passage that is connected in parallel to a passage in which the downstream check valve is arranged, wherein the controller is configured to open and close the regeneration control valve.