The electrochemical measurement called a cathodic reduction method is a type of galvanostatic electrolysis. The cathodic reduction method was applied to determine the film thickness of copper oxide and copper sulfide. Plateaus are appeared in the potential vs. time curves when the commercial reagent-grade purity powder of cuprous oxide （Cu2O）, cupric oxide （CuO）, and cuprous sulfide （Cu2S） were cathodically reduced at a constant current density of −1mA/cm2. The plateau potentials of cuprous oxide, cupric oxide and cuprous sulfide are −0.7, −0.8, −1.15V vs.Ag/AgCl, respectively. A film formed on copper plate immersed in 1000 ppm NaCl and 1000 ppm Na2S solutions has been assessed qualitatively and quantitatively using cathodic reduction method. Cathodic reduction curves revealed that the films formed in NaCl and Na2S solutions were mainly composed of cuprous oxide and cuprous sulfide, respectively. The film thickness calculated from the cathodic reduction curves shows good agreement with the film thickness obtained from cross-sectional inspection.