To detect the rotary speed of an object in fluid and the flow velocity of liquid flowing in one direction.

When an input electrical signal with a carrier frequency f0 is applied to a reed screen-shaped input electrode 2 for input, a leakage elastic wave is excited on a substrate 1. The leakage elastic wave is applied into the liquid in contact with the substrate 1 as a longitudinal wave, and the longitudinal wave is reflected by an object included in the liquid and then is converted to a delay electrical signal with a Doppler displacement frequency (f) corresponding to the movement state of the object by the reed screen-shaped output electrode 3. The traveling speed of the object is related by a frequency difference Δf between f0 and f, thus determining the rotary speed of the object in the fluid and the flow velocity of flowing fluid.