A method of producing an oxidized compound is disclosed. An electric potential is measured of an oxidation reaction solution for producing an oxidized compound by an oxidation reaction, and an end point is determined of the oxidation reaction on the basis of a predefined decrease of the electric potential. Preferably, after the electric potential of the oxidation reaction solution is reached to a highest electric potential, a point is determined where the amount of an electric potential dropped from the highest electric potential reaches the predefined amount of an electric potential as the end point of the oxidation reaction. The method also may introduce an oxidation reaction terminating agent to the oxidation reaction solution immediately after determining the end point of the oxidation reaction.