Provided is a center pillar constituting workpiece, which is equipped with a criterion excellent in the visibility of a spot-welding hitting position thereby to facilitate the welding quality management. A center pillar (3) is constituted of a center pillar outer (5) and a center pillar inner (6), which are jointed to each other by spot-welding a flange (8) formed at the center pillar inner, to the corresponding portion of the center pillar outer. The flange has its edge portion formed into a waving shape having a crest (8a) as a criterion of the hitting position for the spot-welding. The surface corrugations cannot be enlarged so much because of the thickness, but can be formed along the flange face, so that their crests (or valleys) can be enlarged. As a result, the criterion can be made so highly visible as to perform the spot welding at the precise positions according to the designs and to improve the quality of the spot welding.