To obtain stable idling operation of an engine by preventing the inflow of blow-back fuel into an air screw passage and a starter passage.

A single idle air control passage 5 is opened toward an intake passage 2a on the downstream side form a throttle valve 4. A starter passage 6 and an air screw passage 7 are independently branched off from the upstream side passage 5a of the single idle air control passage 5. The upstream side of the starter passage 6 is communicated with air. The downstream side is communicated with the upstream side passage 5a of the idle air control passage 5. The starter passage 6 is provided with a start opening/closing valve 10 for controlling an opening area of the passage. The upstream side of the air screw passage 7 is communicated with air. The downstream side is communicated with the upstream side passage 5a of the idle air control passage 5. An air screw 11 for controlling the opening area of the passage is threadedly attached on the air screw passage.