PURPOSE: To inject a leakage detection material into a closed loop system in operation, by loading a material having a selected amount and concentration in a closed container, and supplying a high pressure system carrier fluid available externally to form a high pressure mixture.

CONSTITUTION: An adjusting needle valve mechanism 44 is slowly opened by turning an adjusting knob 46 to permit a high pressure system carrier fluid to be loaded to a mist injection apparatus. A service man can observe contact of the fluid with a fluorescent die in a container internal space 30 through a transparent protective shield 16 and a container 18. An injection amount of a mixture of the system carrier fluid and the fluorescent dye is controlled with the adjusting knob 46. The adjusting knob 46 is turned and the adjusting needle valve mechanism 44 is closed. Thereafter, a quick separation/mounting element 62 is removed from a system service port. The fluorescent dye is circulated in a closed loop system for several minutes, and then check of a leakage position is achieved using an ultraviolet ray source for detecting a trace of a fluorescent material.