Our recent works on preparation of solid superacid catalysts, sulfate-metal oxides and tungsten or molybdenum oxide supported on zirconia, are reviewed. Solid superacids with an acid strength of up to Ho ≤ -16.04 are obtained by adsorbing sulfate ion onto hydroxides or amorphous oxides of Fe, Ti, Zr, Hf, Sn, and Si followed by calcination in air above 500°C; a superacid of Al₂O₃ is prepared from the crystallized oxide. Superacids by metal oxides with an acid strength of up to Ho ≤ -14.52 are synthesized in the same manner as those of the sulfate superacids by supporting ZrO₂ with WO₃ or MoO₃: impregnation of amorphous ZrO₂ with aqueous ammonium metatungstate or molybdic acid dissolved in ammonia water followed by evaporation of water and calcination in air at 800-850°C. The superacids are satisfactorily active in a heterogeneous system for reactions which are generally catalyzed by strong acid. Structure of acid sites is discussed on the basis of the results of XRD, XPS, and IR spectra.