To obtain an ultrasonic flowmeter preventing from the beginning the generation of an measurement error generated when making measuring sensitivity faster (high-resolution) than a conventional one.

An ultrasonic flowmeter 200 comprises an ultrasonic probe 21 attached/detached to/from piping (a flow channel) 20 which is a measured path in which fluid circulates, and a control portion 30 connected to the ultrasonic probe 21. The control portion 30 has transmission wave oscillation portions 31,39, receiving portions 32, 40, filter portions 33, 41, amplifying portions 34, 42, normalization portions 35, 43, a high-speed computing portion 36, a flow rate calculating portion 37, and a CPU 37. The CPU 37 has a time measuring portion 37a measuring response time of the ultrasonic wave and capable of correcting that, and a flow rate calculating portion 37b capable of calculating the flow rate of the fluid by using the value of the responce time of the ultrasonic wave, etc.