To prevent closure of an air hole from being occluded owing to the biting-in of a foreign substance.

An air valve 1 guides lifting and lowering of a idling valve body 7 and a float valve body 8 with a guide cylinder member 9, lifts the floating valve body 7 via the float valve body 8 with water flowed from a 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 an lower end to close. Between the valve box 2 and the guide cylinder member 9, a circumferential strainer 21 for removing the foreign substance in fluid flowing from the communicating hole 5 into a communicating hole 10 is provided. The strainer 21 is comprised of a capturing cylinder 22 made of metal in which many openings are formed, and caps 23 and 24 installed along a edge of the capturing cylinder 22 coming in contact with the valve box 2 and the guide cylinder member 9.