In the piercing rolling using a plug having a protective coating film formed on the surface thereof, a lubricant agent that is an aqueous solution containing one or more of boron oxide, boric acid and a boron amine salt in the total amount of 5-30 mass% is applied onto the protective coating film on the surface of the plug and the resulting plug is used for the piercing rolling, whereby it becomes possible to improve the service life of the piercing rolling plug reliably without deteriorating the efficiency of the piercing rolling and, consequently, it becomes possible to manufacture a seamless steel pipe with high efficiency. It is preferred that the plug is cooled after the piercing rolling, the lubricant agent is applied while keeping the temperature of the surface of a tip of the plug at 100 DEG C or lower, and the resulting plug is used again for the piercing rolling. In the constitution, it is preferred that the protective coating film on the surface of the plug is formed by the arc spraying of an iron wire material onto the surface of the matrix of the plug and is composed of an oxide and Fe.