PURPOSE: To elevate work efficiency and accuracy of products by fixing the corner of bent plate by welding using a spot welding device, forming the corner by a forming device to facilitate welding of butted part in the following process.

CONSTITUTION: When two corners of a bent plate 5 are suspended on left and right does 23, 23, and an operation button is depressed, the left head 37' moves to the left and the plate 5 comes into contact with the left and right dies 23, 23 and supported. Then, left and right plate pressing cylinders 25, 27 operate and fix the plate 5 from horizontal direction and from above to the dies 23, 23. Successively, a spot welding devicd 73 spot welds the superposed parts at the corner of the plate 5. Next, left and right coining cylinders 35 stretch and a punch 29 presses the butted part between the dies 23 and makes r-bending forming. Then, cylinders 35, 25, 27 retreat and the left head 37' returns to original state. The plate 5 is removed from the dies 23, 23 and attached after turning upside down, and similar work is performed. After completion of working of four corners, the plate 5 is sent to a corner welding device.