PURPOSE: To improve volumetric efficiency by connecting the collecting part, etc. of a suction manifold to a volumetric part through a closing valve, in a system in which said suction manifold is divided according to a plural number of cylinder groups formed by collecting cylinders which do not ajoin each other in regard to their suction order.

CONSTITUTION: When applied to a six-cylinder engine in which the suction order of each cylinder is, #1-#5-#3-#6-#2-#4, the cylinders are grouped in a first cylinder group 2a comprising #1 to #3 cylinders and a second cylinder group 2b comprising #4 to #6 cylinders. The upper course sides of the collecting parts 4a, 4b of the divided suction manifolds 2a, 2b are extended close to an air cleaner 6 through suction passage 5a, 5b and joined at a confluent part 7. A volumetric part 12 is provided between the collecting parts 4a, 4b and the confluent part 7, and each collecting part 4a, 4b and each suction pipe 5a, 5b is connected to the volumetric part 12 through openings 13a, 13b and 14a, 14b which are opened and closed by means of valves 15a, 15b and 16a, 16b respectively.