Bending method and mandrel of multi-layered pipe

A bending method for a multi-layered pipe having one or a plurality of outer pipes disposed concentrically on the outside of an inner pipe, in which an inside mandrel is inserted into said inner pipe of said multi-layered pipe, a tubular outside mandrel is inserted into the annular space between said pipe positioned on the inside and the pipe positioned on the outside thereof, and with the mandrels placed inside said multi-layered pipe, the outermost pipe of said multi-layered pipe is clamped to a bending die by a clamping die, said multi-layered pipe is held at the rear of a bending portion of said multi-layered pipe by a pressure die and a crease-removing die, and said multi-layered pipe is bent by causing said clamping die to revolve around said bending die,

• wherein an annular outside mandrel having at least three slits formed in the distal end thereof along generating lines is used as said outside mandrel, and said clamping die is caused to revolve around said bending die with the distal ends of said outside mandrel and said inside mandrel positioned so as to protrude toward the distal end of said pipe from a bending start point.