PURPOSE: To join efficiently both rolling stocks together by deforming by compression the overlapped parts of the stocks, inserted in between a pair of upper and lower dies, formed by arranging their projected and recessed parts oppositely in a state of mutually engaging state, into recessed and projected shapes.

CONSTITUTION: When the front-end part of a succeeding sheet bar S1 is overlapped on the rear-end part of a preceding sheet bar S2 by a prescribed length, the feeding speed is controlled to feed both bars S1, S2 synchronously. When the front end of bar S1 arrives at the position extremely close to rotary dies 5, 6, a detecting device detects it to rotate the upper and lower rotary dies 5, 6 by one revolution by a driving device. The overlapped part is bitten by the projected parts 5a, 6a and the recessed parts 5b, 6b of the upper and lower rotary dies 5, 6, and is deformed into a projected and recessed shape by compression to be mutually pressure-welded. In this case, both stocks S1, S2 are pressure- welded to each other, while peeling off scales by a large slip produced between the contact surfaces of the shoulder parts of projecting and recessing deformations.