The invention provides an ultrasonic flow velocity meter that can measure the flow velocity of a fluid over a wide range of degree of contamination with high degree of accuracy. The ultrasonic flow velocity meter includes a mounting base to be fixed to the outside of a conduit line in which the fluid to be measured flows, the mounting base having a mounting surface formed obliquely with respect to a longitudinal axis of the conduit line, a sending transducer fixed to the mounting base for emitting an ultrasonic wave to the interior of the fluid, a receiving transducer fixed to the mounting base and receiving the reflected ultrasonic wave having subjected to Doppler shift and emitted from the interior of the fluid, and a calculating unit for calculating the flow velocity of the fluid from the amount of the Doppler shift of the reflected wave received by the receiving transducer. The sending transducer and the receiving transducer are being arranged so that an axis of sending and an axis of reception are coincided with respect to each other, continuously send the ultrasonic wave and continuously receive the ultrasonic wave, respectively.