To prevent foaming efficiency from decreasing even when an emission flow rate decreases.

A mixed fluid A containing a foam undiluted solution, the water, and the air is flown into a gap 6 from a through-hole 32 opening to a circumferential part of a fixing part 3 and flown out to a downstream side from a through-hole 42 opening to a central part of a movable part 4. A downstream side of the movable part 4 is energized by a coil spring 5 in the direction of the fixing part 3. The width of the gap 6 is the width in which push-in pressure of the mixed fluid A into the gap and one adding an energizing force of the coil spring 5 to the downstream side pressure of the movable part 4 are balanced. In the case of normal flowing flow rate, the width of the gap is set so that flow speed in the gap 6 is fast enough and powerful turbulent flow occurs. Flow speed in the gap 6 decreases as the emission flow rate decreases, but the push-in pressure of the mixed fluid A into the gap decreases, the gap becomes narrower, and a decrease of flow speed in the gap is offset.