An air bypass valve comprising: a coil casing in which a coil is accommodated; a pilot valve section covered by the coil casing, the pilot valve section comprising a movable iron core inserted into the coil; and a piston section fitted in the coil casing, the piston section comprising a piston having a hole at a bottom thereof; characterized in that the piston section comprises: the piston; a first spring accommodated in the piston; and a piston casing having an opening portion formed to cover an outer periphery of the piston, the pilot valve section comprises: the coil in which a copper wire is wound around a hollow cylindrical bobbin; a fixed iron core inserted into an inner diametrical part of the coil from one end side thereof; the movable iron core inserted into the inner diametrical part of the coil from the other end side thereof, the movable iron core comprising recesses at both ends; a yoke inserted into a clearance between the inner diametrical part of the coil and the movable iron core; a second spring accommodated in the recess on one end side of the movable iron core; a steel ball accommodated in the recess on the other end side of the movable iron core; a valve seat disposed so as to face the other end side of the movable iron core; and a seat holder having a through-hole, the valve seat and the steel ball being accommodated in the seat holder, and at least three protrusions are provided at an edge of the opening portion of the piston casing. a coil casing in which a coil is accommodated; a pilot valve section covered by the coil casing, the pilot valve section comprising a movable iron core inserted into the coil; and a piston section fitted in the coil casing, the piston section comprising a piston having a hole at a bottom thereof; characterized in that the piston section comprises: the piston; a first spring accommodated in the piston; and a piston casing having an opening portion formed to cover an outer periphery of the piston, the piston; a first spring accommodated in the piston; and a piston casing having an opening portion formed to cover an outer periphery of the piston, the pilot valve section comprises: the coil in which a copper wire is wound around a hollow cylindrical bobbin; a fixed iron core inserted into an inner diametrical part of the coil from one end side thereof; the movable iron core inserted into the inner diametrical part of the coil from the other end side thereof, the movable iron core comprising recesses at both ends; a yoke inserted into a clearance between the inner diametrical part of the coil and the movable iron core; a second spring accommodated in the recess on one end side of the movable iron core; a steel ball accommodated in the recess on the other end side of the movable iron core; a valve seat disposed so as to face the other end side of the movable iron core; and a seat holder having a through-hole, the valve seat and the steel ball being accommodated in the seat holder, and the coil in which a copper wire is wound around a hollow cylindrical bobbin; a fixed iron core inserted into an inner diametrical part of the coil from one end side thereof; the movable iron core inserted into the inner diametrical part of the coil from the other end side thereof, the movable iron core comprising recesses at both ends; a yoke inserted into a clearance between the inner diametrical part of the coil and the movable iron core; a second spring accommodated in the recess on one end side of the movable iron core; a steel ball accommodated in the recess on the other end side of the movable iron core; a valve seat disposed so as to face the other end side of the movable iron core; and a seat holder having a through-hole, the valve seat and the steel ball being accommodated in the seat holder, and at least three protrusions are provided at an edge of the opening portion of the piston casing.