To provide a method for easily and efficiently producing the subject compound useful as a raw material for a synthetic resin, e.g. polyester resin in a moderate condition, by contacting a specific aromatic condensed cyclic compound with a molecular oxygen in the presence of an oxidation catalyst comprising a specific imide compound.

This compound (e.g. naphthalic acid) of formula III is obtained by contacting (B) an aromatic condensed cyclic compound (pref., acenaphthene) of formula II (R3 to R8 are each H or the like; and (m) is 0 or 1)with (C) an molecular oxygen in the presence of (A) an oxidation catalyst comprising an imide compound (e.g. N-hydroxyphthalimide) of formula I (R1 and R2 are each H or the like; X is oxygen or the like; and (n) is 1 to 3) pref. in the proportion of about 0.05 to 0.25 mol. of the component A and about 4 to 100 mol. of the component C based on 1 mol. of the component B, ordinarily in an inactive organic solvent e.g. acetic acid, pref. at 50 to 200°C, under about 1 to 50 atmospheric condition, pref. for about 2 to 24 hrs.