To reduce the deformation of a core caused by high temp. at the time of casting and to enable the simplification of removal of the core with chemical leaching after casting by using the reinforcing core material arranging carbon base reinforcing material in the inside of a tubular sheath consisting essentially of a qualtz or the other silica and forming the bent core.

The sheaths at the outside forming the core material are formed to about 0.010-0.015 inch wall thickness by etching with hydrofluoric acid water solution to remove the surface defect. This shape can be used as triangle columnar shape, etc., besides a round columnar shape and is decided so as to match to the prescribed shape of a casting together with a pipe diameter and length. Further, the carbon base reinforcing material 4 inserted along the inside and the longitudinal direction of the sheaths 1, 2 is made of graphite rod or carbon fiber flux, and the shape and size, etc., are decided with the shape of the sheaths 1, 2. At the time of forming to the desired bent shape by jointing the complex core material, after joining the carbon base reinforcing material 4 at the joining point 3 by using carbon joining material etc., the sheaths 1, 2 are melt-joined and the continuation of the inner part passages 1a, 2a is held.