PURPOSE: To provide a high output without increasing suction resistance, by a method wherein an expansion chamber is formed in the middle of a suction passage, and a connection pipe moved forward and backward to and from the one of the open air side and the engine side of a suction passage opened in an expansion chamber is connected to the other of the open air side and the engine side.

CONSTITUTION: A branch pipe branched from each of a plurality of cylinders in a suction passage is formed with the open air side 7e and the engine side 7f with an expansion chamber 16 therebetween. A connection pipe 17, moved forward and backward to and from the open air side 7e, is slidably loosely engaged with the inner side of the engine side 7f. The connection pipe 17 intercouples adjoining two cylinders by means of a coupling member 18, and drive is effected by means of a negative pressure differentiating means 19. Namely, during low speed low load running of an engine, a low suction pipe negative pressure is exerted on one chamber 24 through a modulator 25, and by rightward protrusion of the connection pipe 17 by means of a resilient film 21, the suction passage is separated from the expansion chamber 16, and the open air side 7e is connected to the engine side 7f.