Flow rate measurement is performed with a propagation time by using a pair of ultrasonic wave oscillators which is provided in flow path through which a fluid to be measured flows, and is disposed so as to transmit and receive an ultrasonic wave signal by causing the ultrasonic wave signal to be reflected on an inner wall of flow path at least once. In addition, adjustment of an amplification rate is periodically performed by amplifier, which amplifies the ultrasonic wave signal received by each of the ultrasonic wave oscillators to a predetermined amplitude, and a difference between a previous amplification rate and a current amplification rate is a predetermined value or more, and the instantaneous flow rate calculated by flow rate calculation unit is a predetermined flow rate or less, the reference voltage is adjusted by reference voltage setting unit. With the configuration, the propagation time can be stably measured and deterioration of the maximum amplitude is prevented.