An actuator for an air valve placed in the boost air duct of an IC engine and, more specially, of such an engine supercharged or boosted by a pressure wave supercharger, the air valve being in its closed position on starting up the IC engine from cold, at which time the engine takes in charge air by way of a breather valve, and at the end of the starting stage the valve in the boost air duct is moved into its open position and is kept open while the engine is running. After shutting down the engine, the valve in the intake pipe is automatically moved back into its closed position and the air valve is moved into its open position by the pressure of the air forced into the boost air duct and/or by vacuum against an air valve spring closing force.