Welding device and method for manufacturing resin welded workpiece

A method for manufacturing a resin welded workpiece in which a resin portion is welded to a workpiece, the method comprising:

• a welding process in which the resin welded workpiece is manufactured by melting the resin portion placed on the workpiece and pressing the resin portion having been melted against the workpiece via a sheet material with a pressing member to weld the resin portion to the workpiece;
• a moving process in which the sheet material is slid in a moving direction of the sheet material when the welding process is performed at least once, the moving direction intersecting a pressing direction in which the resin portion is pressed against the workpiece in the welding process;
• and
• a moving determination process in which whether the sheet material is to be slid is determined based on a number of welding processes endured by the sheet material with respect to a predetermined threshold, the predetermined threshold being set based on a degree of deterioration of the sheet material, wherein
• the moving process is performed when the number of welding processes endured by the sheet material corresponds to the predetermined threshold,
• a region of the sheet material that is pressed by the pressing member in the welding process is defined as a sheet pressing region, and
• in the moving process, a region of the sheet material different, in the moving direction, from the sheet pressing region in the welding process before the moving process is slid to a position that is to be pressed in the welding process after the moving process.