UL400 Ultrasonic flowmeter by using the duration difference of ultrasonic wave transferred in moving fluid is capable of measuring a flow rate of metallic and plastic piping ranging from 25 to 400 mm in nominal diameter. A detector (an ultrasonic sensor) is mounted outside an existing piping by clamping method, so that it does not get into contact with the measuring fluid at all, and there are no concerns about the mixture of solid material and metallic ion into the fluid, the corrosion of sensor by chemical, and the pressure loss by installing the flow meter.