PURPOSE: To improve the volumetric efficiency under inertial supercharging, by connecting one primary passage of an intake manifold to the other along a curve and prescribing the ratio of the radius of the inside of the curve to the inside diameter of the former primary passage.

CONSTITUTION: The action of a turbocharger 35 is matched in the rapid rotattion range of an engine 31. Inertial supercharging is matched in the slow or medium rotation range of the engine. When a valve 40 is closed, matching is effected in the slow rotation range of the engine 31 because the characteristic frequency of a tube system including a main pipe 34 and extending downstream therefrom is low. When the valve 40 is open, matching is effected in the medium rotation range of the engine because a tube system including a bypass pipe 39 and extending downstream therefrom vibrates and has a higher characteristic frequency. The volumetric efficiency ηv is high when the inside diameter D of primary pipes 33A, 33B and the radius R of curvature of the inside of a part coupling the main pipe 34 to the primary pipes are prescribed as R/D≥0.05.