Disclosed is a flow rate measurement device, comprising a time differential memory unit (17b), which stores a time differential of a propagation time of an ultrasonic signal in a forward and a reverse direction that is measured in a unit measurement process, while switching a transmission and reception direction in a forward and a reverse direction of two oscillation elements (2, 3) when carryingout each respective K iterations of the unit measurement process that measures the propagation time in the forward and the reverse direction thereof, a flow rate computation unit (15), which derives a pass-through flow rate of a liquid body, based on a sum of the propagation time in the forward and the reverse direction of the unit measurement process that is carried out not less than K iterations thereof, and an estimation unit (18), which estimates an instantaneous change in the flow rate of the liquid body based on the time differential for each of the K iterations of the unit measurement process that is stored upon the time differential memory unit (17b), thereby facilitating a detection of the instantaneous change of the flow rate while deriving an accurate pass-through flow rate.