PURPOSE: To obtain a good internal face quality by deciding the moving speed of a mandrel bar and the initial position of the rolling start time under the conditions of reaching the start point of a roll bite part simultaneously with the rear end of a blank pipe and the effective part of the mandrel ber.

CONSTITUTION: Movement is started from the rolling start time when the front end of a blank pipe H reaches the start point of a roll bite and is engaged between a thick rolling reduction part 1a and mandrel bar 2. It is decided so that the front end in the moving direction of the effective part of the mandrel bar 2 may reach the terminal of the roll bite part together with the rear end of the blank pipe H. Moreover, the moving speed of the mandrel bar 2 initially positioned as above is decided so that the effective part rear end may reach the start point of the roll bite part simultaneously with the rear end of the blank pipe H. Thus the whole length of the effective part of the mandrel bar is brought into contact with the pipe, there is no part contributing to no rolling of the pipe, priortity for the thermal load and lubricity while under rolling is caused and a high internal face quality can be obtd.