An air intake system for an internal combustion engine has a valve body that can open/close a passage cross section of a part of an air intake passage; a rotation shaft that extends to both sides of the valve body and supports rotation of the valve body; a valve accommodating recess that is formed on an inner wall surface of the air intake passage and accommodates therein the valve body; and a protruding line that is formed on an outer peripheral side surface of the valve body, which faces the valve accommodating recess, and extends along a shaft direction of the rotation shaft. The air intake system further has a communication passage that directly connects the valve accommodating recess and an intake port of the internal combustion engine.