PURPOSE: To improve the combustion performance by enhancing the output torque at the time of full load throughout the operating ranges, i.e. in high, middle and low numbers of engine revolutions, and by providing a better suction swirling at the time of partial loading, and also to offer stabilized lean combustion by forming the suction gas in combustion chamber in layers.

CONSTITUTION: At the time of partial loading (a), the upper edge of a control valve 27 is in contact with a partition wall 6 in the downstream of the divergent point, and the lower edge is in No.1 position contacting the inner wall of No.2 suction passage 2b. At the time of full load in low number of revolutions (b), on the other hand, the upper edge of the control valve 27 is in contact with the inner wall of No.2 suction passage 2b, and the lower edge is in No.2 position contacting the partition wall 6 in the upper stream of the divergent point. At the time of full load in high number of revolutions (c), both the upper and lower edges of the control valve 27 are in No.3 position being free from contact with the partition wall 6 and inner wall of No.2 suction passage 2b.