PURPOSE: To obtain the titled substance useful as an intermediate for synthesizing dye, etc. in high yield for a long time with preparing a small amount of by- products, by oxidizing a diphenylmethane or a benzophenone as a raw material using molybdenum oxide and titanium oxide as a catalyst.

CONSTITUTION: In oxidizing a diphenylmethane shown by the formulaI(R1 is 1W4C alkyl, formyl, carboxyl) or a benzophenone shown by the formula II (R2 is 1W4C alkyl, formyl, carboxyl) as a raw material, it is subjected to contact gaseous oxidation with an oxygen-containing gas in the presence of a catalyst comprising molybdenum oxide and titanium oxide at 200W700°C, preferaby 250W500°C, to give the desired compound. The catalyst may comprise an oxide of calcium, antimony, bismuth, chromium, etc. as a third component as well as molybdenum oxide and titanium oxide as the main components.