An air control device is advantageously used as a device for supplying secondary air to a catalyzer contained in an exhaust pipe of an internal combustion engine. The air control device includes an electric motor, a speed reduction mechanism, a valve having a valve shaft and a biasing spring. These components are all contained in a housing that has a valve port formed therein. The valve is driven to open the valve port by the electric motor against a biasing force of the biasing spring. A rotational movement of a final gear in the speed reduction mechanism is converted into a linear movement of the valve shaft to thereby open the valve port. Since the biasing spring such as a torsion spring is wound around a flange connected to an axis of the final gear, a length of the valve shaft is made short, and the device as a whole is made compact.