An ultrasonic flow meter for measuring a flow rate in a flow path is provided having ultrasonic sensors each of which includes a semicircular or substantially circular disk shaped transducer. Such transducers are detachably mountable on an envelope of the flow path. The thickness of the ultrasonic sensors in the direction of the length of the flow path is reduced to improve a detection resolution. A plurality of the ultrasonic sensors are mounted on the flow path while keeping a predetermined distance therebetween in a length direction of the flow path, so that the flow rate can be measured from the difference of propagation times of ultrasonic waves between the ultrasonic sensors.