To provide a method for verifying ultrasonic feed-water flowmeters capable of heightening the reliability of measurement accuracy of a feed-water flow by a Chordal type ultrasonic feed-water flowmeter, reducing estimated errors in thermal output operations, and improving the operating efficiency of a plant.

A channel for simulating an actual apparatus and an actual ultrasonic flowmeter are used to perform verification by a simulation test in steps 102-104. Verification is separately performed by analysis by an analytical model in steps 105-110. Evaluation results of the simulation test verification step 104 and the analysis verification step 110 are combined and re-evaluated to verify the measurement accuracy of the ultrasonic flowmeter on the safe side in a double verification step 111.