To provide a flat cross-sectional carbon fiber bundle which is excellent in a fire-resisting process-passing property and can form composite materials having high carbon fiber contents, in spite of the flat cross-sectional carbon fiber bundle.

The flat cross-sectional carbon fiber bundle, whose cross-sectional texture is an aggregate of isotropic textures, has a surface smoothness of 1.20-1.5 and a flat cross-sectional shape having a major axis (a)/minor axis (b) ratio R of 1.2-10. A flame-resisting polymer produced from polyacrylonitrile as a starting substance is used for a precursor polymer of the carbon fiber.