The ultrasonic flaw-rate measurement unit of the invention includes a flow measurement path inside which fluid to be measured flows through and to which a case accommodating a pair of ultrasonic transducers is installed, a pair of ultrasonic transducers disposed in the flow measurement path, a measuring circuit substrate incorporating a propagation time measuring part measuring a propagation time between the ultrasonic transducer transmitter and receiver and a flow-rate calculating part calculating a flow rate of the fluid to be measured based on the propagation time measured by the propagation time measuring part, and a damping material filling an interior space of the case and covering the pair of ultrasonic transducers.