To precisely measure a flow rate value with a reduced measuring frequency, even when a flow rate of a measuring object is fluctuated by fluctuation of pressure or the like.

A cycle measuring period and a flow rate measuring period are provided in measurement by a flowmeter, a fluctuation width Δh of a propagation time hfj between a transmission sensor and a reception sensor, and a distortion related value hv of a fluctuation cycle hf of the propagation time are found in the cycle measuring period, those are compared with a prescribed fluctuation width allowance value h0 and a prescribed distortion related allowance value hv0 to be classified into A (low fluctuation), B (sine wave fluctuation) and C (distorted wave fluctuation), and a measuring time interval tT for measuring an ultrasonic flow rate in the flow rate measuring period is changed based on the classification to measure the flow rate.