A flow rate measuring device measures a flow rate of a main flow flowing through a duct. The flow rate measuring device includes a bypass passage, a flow rate sensor, and a measurement body. The measurement body includes a measurement flow inlet opening toward an upstream side of the duct and a cylindrical portion opening toward a downstream side of the duct. The cylindrical portion is disposed to cover the measurement flow outlet. The cylindrical portion includes an outer circumferential surface guiding the main flow and an inner circumferential surface guiding the measurement flow. The cylindrical portion includes a waveform portion formed in an edge side of an opening of the cylindrical. The waveform portion divides the main flow guided by the cylindrical portion into a plurality of main flows and divides the measurement flow guided by the cylindrical portion into a plurality of measurement flows.