PURPOSE: To stabilize the lubrication of a turbocharger bearing part at the suspension side and the function of an oil seal, by changing each turbocharger over to either its small or large capacity one according to an engine speed, while feeding a part of exhaust gases to the turbocharger at the suspension side as well.

CONSTITUTION: At a low speed, almost all of exhaust gases out of an engine 7 are fed to a small capacity turbine 1, while a part of the exhaust gases is fed to a large capacity turbine 4. Air out of a silencer 19 enters the engine 7 by way of a blower outlet side selector valve 29 and a blower outlet side selector valve 25 at a small capacity blower 2. On the other hand, since air out of a large capacity blower 5 is closed and intercepted by the blower outlet side selector valve 25, it circulates a circulating flow passage 32. At a high speed in the engine, selector valves 9, 25 and 30 are selected to each direction so that operation is reversed to that in case of the low speed.