To form an acetal compound, a carbonyl compound, etc., by bringing an ether into contact with nitrogen monoxide in a specific ratio in the presence of a specific oxidation catalyst so as to readily and efficiently oxidize the subject compound.

An ether [e.g. an ether of formula II (Ra1 and Ra2 are each H, a hydrocarbon group, a heterocyclic group, a hydroxy group or a substituted oxy; Rb is a hydrocarbon group or a heterocyclic group)] is brought into contact with ≥5 mols based on 1 mol of the ether of nitrogen monoxide in the presence of an oxidation catalyst composed of an imide compound of formula I (R1 and R2 are each H, a halogen, an alkyl, an aryl, a cycloalkyl, hydroxy an alkoxy, carboxy or the like or R1 and R2 are mutually bonded to form a double bond or a ring; X is oxygen or hydroxy; n is 1-3) (e.g. N-hydroxyphthalic imide, etc.).