A dust seal structure for a valve stem in a rotary throttle valve The dust seal structure has a cylindrical throttle valve with a metering needle and a throttle through-hole, and a fuel nozzle disposed on the central axis of the throttle valve that is openable into the throttle through-hole with the metering needle inserted therein. The valve stem of the throttle valve, which protrudes at one end from the shaft hole formed in the cover body of a valve hole in the carburetor body, is caused to rotate in accordance with the operation of the accelerator pedal, whereby the throttle valve moves in the direction of a central axis thereof while rotating as a whole, and controls airflow and fuel-flow rate. A dust-proof member composed of a sealing ring is fitted between the valve stem and the shaft hole formed in the cover body.