PURPOSE: To operate an engine with good efficiency of a supercharger by bypassing compressed air for driving a power recovery air motor during low-load operation to the turbine side.

CONSTITUTION: During high-load operation, a control valve 14 is opened, and a control valve 13 is closed. Some of air fed from a compressor 3 to a Diesel engine 4 is introduced to an air motor 9 to drive the air motor 9, whereby power is recovered through a shaft 10. When the load of the Diesel engine 4 is lowered to be put in the state of low-load operation, the control valve 13 is opened and the control valve 14 is closed. As all the compressed air used for operation of the air motor 9 is introduced from a discharge side line 76 through a by-pass line 12 into an exhaust gas line 6, the horse power of the turbine 2 is increased. As some of the compressed air is used for driving the air motor 9, air supply pressure from high-load to low load is gently raised.