PURPOSE: To measure the accurate flow speed and flow rate all the time, by the constitution, wherein the axis of an ultrasonic wave beam from a transmitting transducer is approximately common to the axis of a receiving transducer having the receiving directivity.

CONSTITUTION: A transmitted signal from a signal oscillator 1 is converted into an ultrasonic wave signal. The signal is transmitted by a transmitting transducer 2 as an ultrasonic signal beam into a fluid to be measured 5 in a pipe 4. The transmitting transducer 2 is arranged on the wall of the pipe 4. Meanwhile, solid state particles 8 in the fluid to be measured 5 reflect said ultrasonic wave beam. A receiving transducer 11 receives the reflected wave and converts the wave into an electric signal. The receiving transducer 11 is arranged at the neighboring position to the transmitting transducer 2 so that an axis 7 of the ultrasonic wave signal beam is made common to an axis 7a of the reflected wave having the receiving directivity. Thus the accurate flow speed and flow rate can be always measured regardles of the pollution concentration of the fluid to be measured.