PURPOSE: To obtain superior shape retentivity and suppress secular change by imparting bending habits to a sheet blank material for bellows with a high-frequency welder or the like.

CONSTITUTION: A sheet blank material 1 is made essentially of, for example, vinyl chloride, and its rear surface has been treated to a matte surface. This sheet is beforehand cut to a rectangular shape. The bending lines 2 (2a) to become the seams for folding so as to be made into bellows are formed by inscribing with a high-frequency welder 4 having plural high-frequency electrodes from above after placing the sheet blank material 1 on a buffer 3. The bending lines 2a formed in this way are of a groove shape and this results in that the bending habits of a V-shape are given along these bending lines 2.