To generate a cut surface of fine workmanship when a branch pipe is cut from a main pipe.

A pipe device is configured so that an edge preparation 12 is formed at a branch pipe 10 and the intersecting point between the edge preparation 12 and the inside line of the branch pipe 10 is put in point contact with the outside line of a hole in a main pipe 11. According to the cutting method with this pipe device, the cutting angle of the branch pipe 10 is varied, and the edge preparation angle of the branch pipe 10 to the outside line of the main pipe 11 is maintained constant.