To inspect safely the accuracy of a pressure gauge mounted to the equipment piping of noxious-gas.

A noxious gas cylinder is removed from the equipment piping to connect a pressure gauge accuracy inspecting apparatus 20 having a small tank 22 and large tank 24 to the equipment piping. Valves 5, 10 of the equipment piping are opened so that the small and large tanks 22, 24 are filled with leak checking N2 gas. Next, the N2 gas in the small tank 22 is set to the reference pressure which is composed with the pressure shown by pressure gauges 4, 7 for inspection to inspect whether or not the pressure is within a specified value for the reference pressure. When the reference pressure is changed stepwise for the inspection, an orifice incorporating type solenoid valve 40 and valve 42 of the pressure gauge accuracy inspecting apparatus 20 are closed and an orifice incorporating type solenoid valve 40 in opened. Then, the pressure in the small tank 22 is raised by the pressure in the large tank 24. When the pressure in the small tank 22 reaches the predetermined reference pressure, the orifice incorporating type solenoid valve 40 is closed.