A flow rate measuring apparatus capable of accurately measuring a mass flow rate of an exhaust gas, includes a laser distributor causing a sheet-like laser to move in a laser introduction duct. An imaging device takes images at specified times. A processor calculates, based on the images, motion vectors of an exhaust gas for a period at a plurality of positions in each image; averages the motion vectors into one motion vector; and calculates the Y-components of the averaged motion vectors in the images as movement amounts of the exhaust gas, an arithmetic mean of the movement amounts as a representative movement amount of the exhaust gas, an actual flow velocity of the exhaust gas from the representative movement amount, an actual flow rate of the exhaust gas from the actual flow velocity, and, based on the actual flow rate, a mass flow rate of the exhaust gas.