PURPOSE: To enable the secure improvement of the life of a plug by applying lubricant made from water glass solution after a water film on the surface of the plug vaporized.

CONSTITUTION: In an inclined rolling mill, rolling rolls are arranged with an inclination to the pass line of a rolling stock, and a plug 12 is arranged on the pass line. The plug 12 used for the rolling of one rolling stock is introduced and cooled in a cooling water bath 20 with a plug bar 13, brought in the inclined rolling mill again, and used repeatedly. Accordingly, during cooling the plug 12, the cooling is stopped at a point of time when a surface temperature is 150°C or less and the temperature part of 100°C or over is present inside. And, after a water film on the surface of the plug 12 evaporated, lubricant made from water glass aqueous solution is applied on the surface of the plug 12 with a lubricant applying header 23. Consequently, a lubrication working during rolling can be stabilized.