PURPOSE: To permit definite decision of the corrosion resistance of a zirconium alloy by eliminating the influence of an oxide film of a zirconium alloy test piece at 450W550°C which affects the corrosion of said alloy in steam and is formed at the time of heating up.

CONSTITUTION: After the test piece is mounted to a pressure vessel 1, a valve 8 is closed and valves 9, 10 are opened to pass an inert gas from a cylinder 11 into the vessel 1. The vessel 1 is thereafter heated up to a prescribed test temp. by a heater 4. The valve 10 is held open to keep the inert gas flowing during the heating up or the valve 10 or both the valves 9 and 10 are held closed to hermetically closed the vessel 1 while an inert gaseous atmosphere is maintained therein. The valve 9 is closed and the valves 8, 10 are opened to admit water from a water storage tank 12 through an inflow port 5 into the vessel 1 by a pump 13, when the vessel 1 attains 450W550°C prescribed test temp. The water or steam admitted into the vessel 1 flows therein and after the water or steam flows out 6 of the vessel, the water or steam is cooled 15 and is held under ≥50kg/cm2 by a reducing valve 16. The water or steam is discarded as it is or is returned to the tank 12 so as to be recirculated.