An apparatus for controlling the amount of intake air to an engine comprises a throttle body, a throttle valve disposed in the body, a bypass passage that bypasses the throttle valve. The throttle body forms a part of the air intake passage for the engine. In one embodiment of the invention, a guide passage is formed in the throttle body and extends upwardly from the bypass passage. The open end of the guide passage is disposed above the center line of the inside of the throttle body. In another embodiment of the invention, a barrier such as a protrusion or groove is formed in the inner surface of the throttle body and disposed above the open end of the bypass passage to prevent fuel, oil, and other substances from flowing into the bypass passage.