PURPOSE: To simplify a finishing process and to improve the productivity of an outer panel by projecting the weld zone of two curved sheet metal member outwardly while removing the thermal strain of the weld zone, and polishing the projection part together with a weld impression.

CONSTITUTION: The sheet metal members a1 and a2 of the outer panel (a) are made to overlap each other with an overlap margin 1 provided to the end part of one member; spot welding is performed for temporary fitting and the part is seam-welded. The weld zone 2 is pressed by rolls 3 and 4 to the overall length from the outside and inside in curvature directions to remove the thermal strain. At the same time, the weld zone 2 is projected as a projection streak 5 outwardly in the curvature direction, and the projection streak 5 is grounded away together with the weld impression formed in the weld zone 2.