PURPOSE: To facilitate water circulation by coupling the interior of measuring tank to the suction port of pump while providing a valve for switching the suction port between water storage tank and measuring tank and returning the water from measuring tank to water storage tank through pump upon finish of measurement.

CONSTITUTION: A measuring tank 13 mounted on a scale 12 is provided at the side of water storage tank 11 and a suction pipe 14 lead out from the storage tank 11 is coupled to the suction port of pump P. Valves 18, 19 are arranged respectively in the suction pipe 14 and the branch pipe 17 to switch the suction port of pump P between storage tank 11 and measuring tank 13. Consequently, water can be returned from the measuring tank into the storage tank through pump resulting in facilitation of water circulation.