PURPOSE: To confirm a state and a range of an airtight leak by enclosing a tracer in the high pressure side of a chamber with different pressures and making visible the tracer which injects at the low pressure side by the leak.

CONSTITUTION: A transparent pressure vessels made of resin is divided into the upper chamber 1 and the lower chamber 2 via a flange 3, a partition plate 5 and a seal-rubber ring 6 and keeps the airtightness by inserting a bolt into a bolt hole 4 and tightening it. Then, fine powder of fluorescent dye is enclosed in the compartment chamber at the high pressure side of the pressure vessel via an inlet pipe 7 and a pressure gage 8 in an airtight detection test of the flange 3 and the partition plate 5. When the prescribed pressure and temperature are loaded and the face pressure of the flange 3 is decreased the fluorescent dye is injected from the flange 3 face and stuck to the flange face where the airtight leak takes place in the pressure vessel. Then, when a special light is brought into contact with the leaked fluorescent dye, the fluorescent dye emits the light, and the position and range of the leak are decided.