PURPOSE: To prevent defective welding at the time of performing butt welding on end faces cut with a gradient in the cross direction of preceding and succeed ing beltlike metal plates by cutting both plates separately and rolling a joined part to suppress the occurrence of burrs at the time of cutting.

CONSTITUTION: The preceding and succeeding beltlike metal plates 36 and 38 are fixed by outlet side and inlet side clamps 26 and 22 respectively. After the tip of the plate 36 is cut diagonally with a cutting unit 32 inclined by ≥5°, a joining unit 14 is then slightly moved in the right direction and an end of the plate 38 is cut at the same angle as before by the unit 32. The unit 14 is then moved in the right direction and a welding unit 34 is arranged above the cut ends of the plates 36 and 38 and in this state, the clamps 26 and 22 are operated and the cut parts of the plates 36 and 38 are superposed (or abut ted) slightly on each other and fixed. Afterward, the plates 36 and 38 are welded and joined together by the unit 34 and the unit 14 is moved to the left direction and a rolling unit 30 is arranged above the joined part 48 to roll the joined part 48.