PURPOSE: To smooth the frictional torque in a stage, to heat with good efficiency, to shorten a braking time and to pressure-weld with less temp. fall while in a speed reproduction by making the relative speed of a pair of bodies to be welded with pressure at the initial time of a heating stage larger than the relative speed in the later time.

CONSTITUTION: A bar like stock 10 is held unmovably by a bearing 14, etc. The bar like stock 10 is slowly driven in the axial direction at the time t1 when the rotation of the co-axial bar like stock 12 reaches a high speed relative rotary speed N1. After abutting the opposed faces of the bar like stock 10, 12 the frictional pressure between the both is controlled so as to reach to a pressure P1 slowly. Since there is a phenomenon that the higher the relative rotary speed is, the less the frictional torque becomes, the initial frictional torque does not become big and further is reduced as the time goes by. Thereafter, the frictional torque is increased by reducing the relative rotary speed to a low speed N2 but it is reduced with the time-lapse and increased again at the rotation stoppage. Therefore, in a heating stage the frictional torque is smoothed, the heating is performed with good efficiency and due to the later time relative rotary speed N2 being low the braking time is shortened, the temp. fall under speed reduction is reduced and the pressure welding is well performed.