PURPOSE: To increase the bending strength of a connection part by press-forming a welding connection part and by invaginating the opposing wall face with its center bent into a pipe hole, then by crushing both center bent invagination parts in a facing direction.

CONSTITUTION: A welding connection part 10 is formed by pressing the part in the prescribed length L at both end parts or one end part of a tubular frame 9. The opposing center parts in the width of the upper wall 9A and lower wall 9B are then pressurized to the inner part of a pipe hole 9H by a press die from the external part of said walls 9C, 9D after their forming in the double folding shape in the width direction with their pressing via a die. In this case, the upper and lower walls 9A, 9B are invaginated with center bend into the pipe hole 9H, overlapped by closely fitting to the side walls 9C, 9D and the thickness of the connection part 10 becomes four sheets piling up portion of the pipe thickness. In this thickness increasing forming, the bending strength in the thickness direction of the connection part 10 can be increased.