To improve fusion strength of a slit of a foaming body with certainty compared to a conventional case.

In order to fuse a slit S of a foaming-pipe P, an operator seize a grip 9 for pressurizing an outer periphery of the foaming-pipe P by means of first pressing rollers 5U, 5D. A fusion blade 25 is entered the slit S. When a lever 35 is pressed down, a cam 31 drives a cam 33 for moving a movable plate 23. Second pressing rollers 15U, 15D are rotated through second links 19U, 19D for pressing the foaming-pipe P. One switch 39B of switches 39 are brought into contact with the other 39A. The switches are set ON. The fusion blade 25 is heated for fusing the slit S. The slit S is continuously fused by relatively moving a fuser 1 in respect to the foaming-pipe 1.