An air compressor is driven by a steam engine that generates power using steam. The steam is supplied to the steam engine through a steam supply path, and the steam is exhausted through a steam exhaust path. The steam from the steam engine is supplied to a steam using device through a steam header. The usage load of the steam is monitored by a pressure sensor arranged in the steam header. The compressed air from the air compressor is supplied to a compressed air using device through a compressed air path. The usage load of the compressed air is monitored by a pressure sensor arranged on the compressed air path. The steam supply to the steam engine is controlled based on the usage load of the steam and the usage load of the compressed air.