PURPOSE: To change a plug at the stage when the life of the plug is expired without monitoring the damaged state by measuring the surface temp. at the tip of the plug after rolling and before cooling and changing the plug at the point of time when a measured temp. reaches a prescribed temp. range for changing the plug.

CONSTITUTION: The functions of the following I and II are achieved with a plug change discriminating device 32 which is controlled with a process computer 31. I: About the surface temp. at the tip of the plug after rolling and before cooling, the temp. range of changing the plug i.e., the temp. range where the life of the plug is expired is predetermined. II: At the time of actual rolling, the surface temp. at the tip of the plug after rolling and before cooling is measured by use of a radiation thermometer 33. At the point of time when the measured temp. reaches the temp. range of changing, the plug is automatically changed by operating an automatic plug changing device 34.