To heap and stiffen a fusion-welded part between the resin pipe and the resin coupling, and solve problems of cracks and breakage caused by the concentration of stress in both the joint parts.

The fusion-welding tool is to fix and hold a resin pipe at the time of fusion-welding the resin pipe 1 and resin coupling 5, and the inner peripheral part of a press-fitting length regulation end face of a pair of half-split sleeve-like holding pieces 3A, 3B provided to the tool is formed into diameter- contracted faces 3a1, 3b1 in which the end part of a molten resin coupling 5 can be inserted deeply in the recessed part in the sleeve axial direction as being at its diameter inside.