PURPOSE: To keep off any interference in each intake passage and thereby improve an intake inertial effect by setting up an on-off valve, being free of rotation, in a primary intake passage, while installing a valve hole, opening or closing an interconnecting part for both first and second intake passages according to rotation of this on-off valve, in its peripheral wall.

CONSTITUTION: In an intake manifold 5 set up at the outer side of a cylinder head 1, there are provided with a primary intake passage 11, interconnecting one side of a suction chamber being connected to an intake path 7 of a throttle body 6 fitted with a throttle valve 8 to in intake port 3, and a secondary intake passage 12 interconnecting the other side of the suction chamber 10 to an intermediate part of the primary intake passage 11, respectively. In this case, a rotary valve 13, opening or closing an opening 12a leading to the secondary intake passage 12, is installed in the intermediate part of the primary intake passage 11. Then, this rotary valve 13 is supported via a pair of bearings 14 so as to cause its inner hole 13a to be vertically continued to the primary intake passage 11. In addition, in a peripheral wall of the rotary valve 13 between these bearings 14, there is provided with a valve hole 15 in accord with the opening 12a.