When a raw material graphite film bad in flatness is laminated onto another material, creases and other defects may be caused. In particular, when a graphite film having a large area is laminated, defects such as creases may be often caused. In order to solve such defects, a flatness correction treatment step is performed wherein a raw material graphite film is subjected to heat treatment up to 2000 DEG C. or higher while a pressure is applied thereto. This flatness correction treatment gives a graphite film good in flatness. Furthermore, when the flatness of the raw material graphite film is corrected by use of a thermal expansion of a core, a graphite film small in sagging can be obtained.