PURPOSE:To prolong the life of a plug by using lubricant having a melting point not exceeding a specified temperature in combination with the plug when a steel tube is rolled by a composite plug for rolling the steel tube having the uppermost surface layer containing a specified content of Mo or Mo alloy. CONSTITUTION:A content of Mo low in thermal expansion is desired to be reduced to improve the life of the composite plug in rolling alloy steel and combined use with the lubricant is effective not to lose fouling resistance. The steel tube is rolled at least by the composite plug for rolling steel tube wherein the uppermost surface layer is made from Mo or Mo alloy containing 20-50% in weight ratio. In this case, the steel tube is rolled in combination with the lubricant having a melting point not exceeding 100 deg.C. As the lubricant. inorganic lubricant is good and chloride or phosphate series lubricant is suitable. Thus the life of the plug is sharply improved, and the operating efficiency is improved by reducing unit requirement and the frequency of plug change.