The Mg-Fe composite oxides having various specific surface area and different crystal structure are obtained, depending on the chemical composition and the calcination temperature. When the mixing ratios are set to Mg:Fe = 1:1, 2:1 and 3:1, respectively, Mg-Fe type LDH is mainly formed as a precursor before calcination. Amorphous composite oxide is obtained by the calcination of Mg-Fe type LDH at 400°C. Approximately, the specific surface area of Mg-Fe composite oxides is increasing with an increase in the mixing ratio of Fe. On the other hand, the specific surface area of them also tends to decrease as the calcination temperature increases over 600°C.