To simply bend a tube without crushing it.

This core material 1 consisting of flexible synthetic resin such as polyolefine resin and nylon resin on which elastically deformable expansion parts 3 having dimensions larger than the inside dimensions of the tube 6 are formed in a part of a main body part 2 where at least a part of its outer peripheral surface is abutted on the inside surface of the tube 6 to be bent is inserted into the bending part of the tube 6 so that the core material 1 is abutted on the inside surface of the tube 6, thereby the tube 6 is bent while the core material 1 is inserted.