To manufacture a spiral pipe easily without flexure by providing a pipe supporting assembly disposed on the outside of the pipe in a radial position fixed to the pipe.

In a small diameter pipe, it is difficult to keep a boom rigid when an outer knife 110 is engaged with a pipe 23. In the case of the boom bending in the direction of being separated from the outer knife during a cutting process, an inner knife and the outer knife move in the separating direction so as not to be overlapped to cut the pipe 23. In this case, a supporting sleeve 231 maintains the inner knife and outer knife in the overlap relation during a slitting process by limiting every flexure of the pipe 23. It is desirable to form the supporting sleeve 31 of heat-treated steel.