PURPOSE: To contrive for saving energy for a compressor in such a way that only when rotation at the settled value has been overstepped, variable speed fluid couplings are connected, the compressor is driven by an expansion turbine.

CONSTITUTION: When rotation of an expansion turbine 3 detected by a rotation detector 17 has been overstepped the rotation at the settled value, this output signal is supplied to an actuator 8 and an electromagnetic valve 12. For this sake, the actuator 8 operates so that a splash pipe 9 may move to the position where working oil will be 100%, the electromagnetic valve 12 opens the circuit and starts up the supply of working oil to a working chamber 6. Then, working oil which has been flowing from the side of an impeller 5 to the side of a runner 7 up to now is flowed from the side of the runner 7 to the side of the impeller 5, the compressor 2 and the expansion turbine 3 become entirely connected condition. By this reason, the compressor 2 is driven by the drive of the expansion turbine 3. Such as this, energy saving drive is contrived for the compressor.