PURPOSE: To carry out a small curvature radius bending by setting many cut grooves in the circumferential direction on the outer circumferential part of the mandrel body made of the elastic material fittable to the inner circumferential shape of the sectional area of the work.

CONSTITUTION: The mandrel 11 for bending the hollow work has the mandrel body 14 of the prescribed length made of the elastic material with its sectional outer circumferential shape formed fittable to the sectional inner circumferential shape of the hollow work A. Many pieces of cut grooves 17 in the circumferential direction are provided on the outer circumferential part of the mandrel body 14. In such a way, the deflection property of the mandrel body 14 is improved, so the recession and the wrinkles, etc., is eliminated by using this, and the bent products having the part to be bent with smaller radius of curvature can be manufactured.