To implement a flow rate measurement at the high accuracy in a piping route of a hydraulic power station which can not ensure a predetermined straight pipe length.

A flow rate measurement method implements: the analysis process of implementing the numeric analysis process, and calculating cross-sectional flow speed distribution data in a hydraulic steel pipe at each flow rate; the linear average calculation process of identifying a linear flow speed distribution on a predetermined line across a cross section of the hydraulic steel pipe at each flow rate, and calculating a linear average flow speed value on the predetermined line at each flow rate; the correspondence identification process of identifying a correspondence between the flow rate and the linear average flow speed value on the predetermined line; the actual measurement process of installing an ultrasonic flowmeter in the hydraulic steel pipe, and measuring the linear average flow speed value of a water flow within the hydraulic steel pipe; and the flow rate identification process of comparing the linear average flow speed value measured by the ultrasonic flowmeter with the correspondence on the predetermined line in the same direction as the direction of the ultrasonic flowmeter for measuring the linear average flow speed value among the correspondence between the flow rate and the linear average flow speed value on the predetermined line, and identifying the corresponding flow rate as the flow rate of the water flow within the hydraulic steel pipe at a location at which the ultrasonic flowmeter is installed.