PURPOSE: To make rapid and proper measure to abnormal rolling possible by deciding that it is normal rolling in the case rolling load is lowered by a certain level of load value after recording a peak value and that other cases are abnormal rolling.

CONSTITUTION: The variation of the pattern of rolling load that is added to rolls at the time of rolling with a restrained mandrel mill 4 is examined. It is decided as a normal rolling in the case rolling load is lowered by a certain level of load value within a fixed time after recording the peak value. Abnormal rolling in the state without a mandrel bar 1 is discriminated by deciding that other cases are abnormal rolling. In this way, effects that are exerted to quality and yield can be restrained at the minimum.