PURPOSE: To stably execute the stripping of a mandrel bar from a base pipe.

CONSTITUTION: In a retractable mandrel mill whose mandrel mill 1 and sizer 4 are arranged in tandem at a distance shorter than the length of the drawn base pipe, outside-diameter sizing rolls 6 are arranged in the final stand of the mandrel mill 1 and rolling for generating a clearance between the mandrel bar 2 and the base pipe without accompanying the reduction of thickness of the base pipe 5. In this way, the mandrel bar is stably stripped with the sizer and overlapping length in which stripping is possible is increased, so the rolling of a long size material is enabled and productivity and yield are improved.