To prevent imperfect block of an air hole owing to the biting-in of a foreign substance without inflowing of the foreign substance reducing an amount of inflow from a small opening with a simple structure.

An air valve 1 guides rise and fall of an idling valve body 7 and a float valve body 8 with a guide cylinder member 9, lifts the idling valve body 7 via the float valve body 8 with water flowed from the small opening 11 formed at a bottom of the guide cylinder member 9 into the guide cylinder member 9, and brings the idling valve body 7 in contact with a valve seat 12 around a big air hole 6 in a valve box 2 having the big air hole 6 communicating with a communicating passage and discharging air at a lower end to close. At a lower part of the small opening 11, there is provided a ring-like support member 32, in which an inflow opening face 32a wider than the small opening 11, by opening a predetermined communicating space 32b downward from the small opening 11, and there is provided with a screen plate 33 at an inflow opening 32a of a support member 32.