PURPOSE: To improve the life of a plug by applying lubricant on the whole surface of a plug before a rolling stock is rolled.

CONSTITUTION: In an inclined rolling mill 10, piercing is performed with upper and lower rolling rolls 11 arranged with a gradient to a pass line and a plug 12 arranged on the pass line. And, a spray header 16 is provided on a bar- steadier chock 15 on the front side of a nearest bar-steadier roll 14 on the outlet side of the inclined rolling mill 10. Lubricant is applied on the whole surface of the plug 12 from the spray header 16. Before the supply of the lubricant by the spray header 16, an air blow for drying water drops on the surface of the plug 12 is performed on the whole surface of the plug 12 with an air blow nozzle 17. Consequently, damage on the surface of the plug at the time of rolling can be reduced.