PURPOSE: To prolong the life of a plug by applying a water-glass base lubricant whose viscosity is specified at a specified temp. on the surface of the plug.

CONSTITUTION: When the plug 12 is situated in front of lubricant applying headers 31, the lubricant is applied on the surface of the plug 12 with the headers 31 after the water film on the surface of the plug 12 is evaporated. An agent consisting of the water-glass base water solution whose viscosity is ≥30P at 1300°C is used as the lubricant. After applying the lubricant, the plug 12 is again situated in front of water shower headers 30 and cooling is restarted. In this way, the lubricant is stably stuck without flowing down from the surface of the plug even to the temp. rise of the plug in rolling and lubrication effect is stably ensured.