Abstract: A flow meter measures a flow rate of fluid flowing through a flow passage based on a duration of propagation of an ultrasonic wave along the flow passage. A signal of the received ultrasonic wave is then compared with a reference voltage to detect an arrival of the ultrasonic wave. A voltage setting unit determines the reference voltage to an appropriate level according to a signal output from a propagation measuring unit which measures a duration of propagation of the ultrasonic wave along the flow passage from the start of transmission of the ultrasonic wave to the output of a signal from a judging unit as the reference voltage being changed. The reference voltage is determined readily and accurately, the flow meter measures the flow rate with the appropriate reference voltage.