PURPOSE: To improve the supercharge efficiency under low speed and transiency by providing a check valve device for blocking the counterflow to an exhaust turbo supercharger side between an intake path collecting section in the downstream of both superchargers and a compressor for an exhaust turbo supercharger.

CONSTITUTION: Under light load low rotation of engine 1, an electromagnetic clutch 20 is turned off to open a relief/bypass valve 20 so as to take the air naturally through a bypath 21 into the engine 1. When the engine 1 is acceleration from light load low or intermediate rotation, the electromagnetic clutch 20 is turned on to close the relief/bypass valve 22, so as to supercharge the air through a second branch path 10b to the engine 1 by means of a mechanical supercharger 19. Since the supercharged intake air from the mechanical supercharger 19 is blocked from counter flowing to a compressor 16a side of an exhaust turbo supercharger 16 by means of a check valve 18, lowering of the supercharged pressure of the mechanical supercharger 19 or lowering of the supercharged pressure of the exhaust turbo supercharger 16 due to the counterflow can be prevented.