An intake system for an internal combustion engine includes: an intake passage (50) through which the intake air is introduced into a combustion chamber (40); an intake manifold (100) that is attached to a cylinder head (30) and forms a part of the intake passage (50); an intake air control valve (200) that reduces the flow passage area of at least a part of the intake passage (50); a valve shaft (300) that supports one end portion (210) of the intake air control valve (200) in a manner such that the intake air control valve (200) is allowed to pivot about the valve shaft (300); and a guide rib (400) that protrudes from the intake manifold (100) toward the one end portion (210), and that minimizes the gap between the one end portion (210) and a portion of the wall surface of the intake passage (50), which is close to the valve shaft (300).