The invention offers a steam turbine forced air cooling system, its method, and a steam turbine provided with the system, the system being of an inexpensive and simple device configuration and improving a cooling effect by the use of an easy-to-get device.Suction is applied to the steam introduction side of an HP turbine 4 or an IP turbine 9 by the use of cooling air suction ejectors 27, 28 which use a compressed medium other than steam as a drive source. The cooling air is then introduced from the steam exhaust portion of the steam turbine into the inside of the steam turbine and is discharged from the ejectors 27, 28 to the atmosphere.