PURPOSE: To positively insert and hold a plug into the top of a tube stock and to prolong the plug service life by forming the tube top part into a truncated cone shape where an inside diameter of the top of the stock is a little larger than the max. outside diameter of the plug and the min. inside diameter of the top inner part of the stock is a little smaller than the max. outside diameter of the plug.

CONSTITUTION: As for a top part 2a of a tube stock 2 with an inside diameter D1 before biting, the part 2a is formed into a truncated cone shape where the outermost top inside diameter D2 is set to be a little larger than the max. diameter d1 of a plug 3 and the min. inside diameter D3 of the inner part is set to be a little smaller than the max. diameter d1. Therefore, generation of rolling troubles is eliminated and shocks at biting by a plug mill are reduced because the plug 3 of the plug mill is positively inserted into and held by the top part 2a of the stock 2.