To provide an air valve capable of improving water stopping property by increasing pressing force of a valve element seat part against a valve casing seat part even when water pressure in the valve casing is decreased by a decrease in water level of a dam.

When the water pressure is generated in the valve casing 2, an excitation starting signal is outputted from a controller 14 to a power source 15 of an electromagnet 12 to turn ON the electromagnet 12, and attracting force of the electromagnetic 12 is made to act on a core part 5A formed by magnetic body of the valve element seat part 5 to enlarge the pressurizing force of the valve element seat part 5 against the valve seat part 6 and improve the water stopping property. When the water pressure becomes negative with existence of air in the valve casing 2, an excitation blocking signal is outputted from the controller 14 to the power source 15 of the electromagnet 12 to turn OFF the electromagnet 12, and a float valve element 4 is made to go down to a valve opening position against energization of a spring 7 by self-weight so as to open an air port 3 and exhaust.