To easily and smoothly swing a paddle without impairing the sealing property, have a troubleproof and simple structure, reduce the labor and cost for maintenance and improve the operation reliability in a paddle type water flow detecting device detecting the water flow by the swinging of a paddle.

This paddle type water flow detecting device 1 has an approximately tubular seal member 15 with a flange insertable around a shaft 6 for preventing water leakage from a fire-fighting distributing pipe 4. The flange part 15a is formed with a slope or stepwise step downward inclined toward the center for forming a clearance S between itself and a washer 17 in a state before being filled with fire-fighting water and applied with water pressure. This sealing structure is so formed that, later when being filled with the fire-fighting water, the flange part around the shaft is deflected by the water pressure to form a dimensional allowance by the deflection and allows the smooth swing of the shaft.