PURPOSE: To secure a state lubricated with fluid so that the scale on the surface of a plug can be protected on the surface of the plug and to prolong the life of the plug by applying a lubricant consisting of a water-glass base water solution of a specified viscosity on the surface of the plug.

CONSTITUTION: When the plug 12 is situated in front of lubricant applying haders 31, the lubricant is applied on the surface of the plug 12 with the headers 31 after a water film on the surface of the plug 12 is evaporated. An agent consisting of the water-glass base water solution whose viscosity is ≤20000P at 1000°C is used as the lubricant. After applicating this lubricant, the plug 12 is again situated in front of the water shower headers 30 and cooling during interruption is restarted. In this way, the prolong of the life of plugis surely realized.