To bend the tube by a simple method without crushing a bending part.

The core material 1 consisting of flexible synthetic resin such as polyolefine resin, nylon resin, etc., which has at least a pair of abutting parts 4 extending in the longitudinal direction and on which recessed parts 9 are provided on both sides of the abutting part 4, is inserted so that the abutting parts 4 are abutted on a face on the bending direction side in the inside surface of the tube 5 and a face opposed to the face respectively in the bending part of the tube 5. After that, the tube 5 is bent in the above direction.