To reduce the amount of air leakage from the air cleaner side of the throttle system into the engine side on an occasion where the throttle valve is set to a closed position, a seal material is installed in the air passage that runs from the upstream side of the suction passage, passes through the upstream side of the gap between the shaft and through hole, communicates with the gap formed by the stepped recession, the shaft and the bearing of the throttle body, runs in the circumferential direction of the shaft and through the downstream side of the gap, and reaches the downstream side of the suction passage.