A crash box has excellent axial crushing performance and in which, even if the sheet thickness of a tubular body constituting the crash box is smaller than 1.4 mm, good welding can be performed with the tubular body butted against a set plate. The crash box (1) having a metallic longer-length tubular body and a method for producing the crash box are provided. The tubular body has a basic cross-sectional shape that is a flat polygon surrounded by a plurality of ridges (2-1 to 2-4) extending in a longitudinal direction and a plurality of side wall portions (4), includes one or more groove portions (3-1 and 3-2) on side wall portions (4) on long sides substantially parallel to the major axis direction of the cross section that extend in longitudinal direction, and includes outward flanges (5-1 to 5-4) in an end portion in the longitudinal direction.