PURPOSE: To restrain the output power of an engine from lowering due to pumping loss, by gradually opening a control valve in the merging section of an intake-air manifold in a rotational speed range exceeding a resonant point during high load operation of the engine so that vacuum parts effected in plural spaces in the merging section is reduced.

CONSTITUTION: When the rotational speed of an engine is increased during high load operation of the engine, a control unit 14 holds a control valve 3 in its closed condition until the rotational speed reaches the resonant point of resonant supercharge in intake-air systems for first and second engine cylinder groups, which are composed of spaces 21A, 21B in the merging section of an intake-air manifold 2 connected to a multi-cylinder engine 1 and resonant type intake-air pipe passages 4, 5. Meanwhile when the engine is operated in a rotational speed range above a rotational speed corresponding to the resonant point, vacuum parts of intake-air pressure vibration effected in the above- mentioned spaces 21A, 21B with respect to the base pressure in a serge tank 6 becomes larger than the positive pressure parts if the control valve is left to be closed. Accordingly, the control valve 3 is opened to decrease such vacuum parts.