PURPOSE: To decrease the damage and deformation of a plug without using a costly high-temp. and high-strength material by disposing the plug and main rolls so as to satisfy a specific relation and executing piercing and rolling while injecting a lubricating agent toward the spacing between the plug and the inside surface of a hollow shell.

CONSTITUTION: The plug of the cross helical piercer having the plural main rolls 1, 2 which are obliquely disposed around a pass line and the plug 5 disposed on the pass line is made into two-stage construction consisting of a head part 5a and a cylindrical part 5b which are constricted at the boundary part. In either case, the diameter d2 of the head part 5b is larger than the diameter d1 of the head part 5a so that the plug can execute thick working of the hollow shell H between the main rolls 1 and 2; in addition, an injection hole 5g for a lubricating oil or coolant is opened to the constricted part. Further, the min. gap g0 of the plug and the main rolls 1, 2 is so set that equation holds to prevent the contact of the hollow shell H with the injection hole 5g. There is, therefore, no need for constituting the entire part of the plug of the costly high-temp. high-strength material.