A composite solid strong acid comprising, a solid acid and a carbon material, wherein said solid acid is obtained by heat treating of polycyclic aromatic hydrocarbons or polycyclic aromatic hydrocarbons to which the carbon material is blended in concentrated sulfuric acid or fuming sulfuric acid, transforming said polycyclic aromatic hydrocarbons to a solid acid which is insoluble in a polar solvent by condensation and sulfonation further compositing with said carbon material.