In a flow rate measurement of large capacity pumps, measuring system like ultrasonic flow meter is used as usual. However, at the pumping stations, the measurement with good accuracy can not be expected owing to the lack of straight part of discharge pipe, etc. Considering such situation, we took notice of the water level of surge-tank which act as the buffer of pressure fluctuation in the discharge pipe. First we identified the loss coefficient of discharge line by measuring the transitional fluctuation of water level in the surge-tank, at the moment of stopping pump from stationary operation, and consequently to know the flow rate only by measuring the difference between the level of river and surge-tank. In this paper, we described the method of deriving the formulation and the results of model test that were carried out to validate the above method.