PURPOSE: To keep the water level in piping constant for accurate measuring when a pressure removing valve is opened.

CONSTITUTION: In this pressure proof expansion test device, the water in a balance water tank 3 is sent to a cylinder 1 side by a pressurizing pump 4 to pressurize and make it expand, and the water is returned to the balance water tank 3 when the pressure is removed by opening a pressure removing valve 10. A portion of piping between the pressure removing valve 10 and the balance water tank 3 is raised higher than the pressure removing valve 10. An air bleeder part 12 is provided on the way of piping between the pressure removing valve 10 and the balance water tank 3. By this, the water level in the piping is kept constant when the pressure removing valve 10 is opened so as to accurately make measurement in pressure proof expansion test with the cylinder 1.