[Problem] A flow rate measuring device in which guided waves are used, wherein the frequency of ultrasound is optimized; and energy injected from ultrasound transmission/reception elements is increased and the flow velocity sensitivity is raised; whereby the measurement accuracy is improved.[Solution] A frequency of an isolated peak of group velocities of guided waves, from among a plurality of peaks of group velocities of guided waves, and a resonance frequency of the ultrasound transmission element/reception element are set to agree; and the semi-amplitude of a power spectrum of ultrasound excited/received by the ultrasound transmission element/reception element is set to a value that does not overlap with another peak of group velocities.