To readily separate a catalyst, products and reaction solvents from a reaction mixture obtained by oxidizing a substance in the presence of the oxidation catalyst composed of an N-hydroxy aromatic imide compound and a transition metal.

This method is to separate the oxidation catalyst, oxidation product and reactional solvent from the reactional mixture obtained by oxidizing the substrate in the presence of the oxidation catalyst composed of the N- hydroxy aromatic imide compound and the transition metal. The reactional solvent is distilled off from the reactional mixture in the presence of an organic solvent immiscible with the reactional solvent to afford a mixture containing the reactional solvent in a weight of ≤5 times based on the N-hydroxy aromatic imide compound. The resultant mixture is then subjected to solid-liquid separation and separated into the oxidation product and the oxidation catalyst.