To check assembling, obviate cleaning of a coolant when assembling failure is caused, facilitate reassembling, and detect an assembling error and a defective part with high accuracy by inspectiong leakage by injecting air into a coupler before the coolant is injected into the coupler.

Before a coolant is filled in a coupler 10, liquid leakage of the coupler 10 is checked. That is, the coolant is injected into the coupler 10 through a diaphragm installing port 7, but before it is injected, an air nozzle 8 is airtightly installed in the diaphragme installing port 7. The air nozzle 8 is connected to an air supply source 21, and pressure is measured by a pressure meter 22. High pressure air 23 is blown into the coupler 10 from the air supply source 21. Airtightness of O rings 19 between a lens 11 and the coupler 10 and airtightness of O rings 19 between the coupler 10 and a face panel 3 of a CRT 1 are sufficient, and when a defect does not exist in respective parts after assembling, indication of the pressure meter 22 does not change. Therefore, airtightness failure after assembling can be easily judged from measurement of the pressure meter 22.