An intake manifold M for an internal combustion engine has two or more structural members P1 to P3 including a first structural member P1 integrally provided with an inlet flange 22 to which a throttle device 4 is connected, and an outlet flange 40 to be joined to a cylinder head 2 provided with intake ports 5. The first structural member P1 is provided integrally with an inlet part 21 forming at least part of an inlet passage 11 and including the inlet flange 22. The outlet flange 40 is formed integrally with the inlet part 21 so as to extend across a direction in which the inlet passage 11 extends, two bolts 451 and 452 for fastening the outlet flange 40 to the cylinder head are disposed respectively on the opposite sides of the inlet passage 11. The rigidity of the inlet flange and fastening of the outlet flange to the cylinder head by fastening means improves a vibration reducing effect on a throttle device joined to the inlet flange.