A water amount measurement device includes a light emitting element to emit light, a light receiving element to receive the light reflected by a liquid surface and by a liquid bottom at a bottom surface within a pipeline, a storage to store a distance from the light emitting element to the liquid bottom, and a processor. The processor computes a water level of the liquid flowing through the pipeline according to different formulas depending on whether a comparison result of distance data obtained based on an output signal of the light receiving element, and the distance at a time of measurement, includes a component corresponding to a distance exceeding the distance generated by the reflection from the liquid bottom, or the comparison result includes a component corresponding to a distance less than the distance generated by the reflection from the liquid surface.