To provide an ultrasonic wave flowmeter detecting measurement errors and failures caused by changes in characteristics of ultrasonic wave oscillators.

The ultrasonic wave flowmeter measuring a flow rate of a fluid passing through a flow channel based on a propagation time difference of an ultrasonic wave comprises at least two ultrasonic wave oscillators 1, 2 transmitting/receiving the ultrasonic wave, and a self-diagnosing means 40. The means 40 detects a first characteristic difference indicating the characteristic difference between the two ultrasonic wave oscillators 1, 2, and/or a second characteristic difference indicating the difference between the past characteristic and the current characteristic of at least either one of the two ultrasonic wave oscillators. Further, the means 40 generates a diagnosis signal indicating the status of the flowmeter based on the first characteristic difference and/or the second characteristic difference.