When driving an air compressor using a steam engine, not only the usage load of compressed air but also that of steam is considered for control. The air compressor (3) is driven by the steam engine (2) which generates motive power using the steam. The steam engine (2) is supplied with the steam through a steam supply path (8) and discharges the steam through a steam discharge path (11). The steam from the steam engine (2) is supplied to a steam using device through a steam header (12). The usage load of the steam is monitored by a pressure sensor (16) provided at the steam header (12). The compressed air from the air compressor (3) is supplied to a compressed air using device through a compressed air path (17). The usage load of the compressed air is monitored by a pressure sensor (18) provided at the compressed air path (17). On the basis of the usage load of the steam and that of the compressed air, supply of the steam to the steam engine (2) is controlled.
