An electrode made of a metal or carbon, a working electrode (20) in contact with this electrode and of a microorganic germ-containing thin film, a counter electrode (7), and a reference electrode (5) as necessary are immersed in an organism-containing solution (30), and the BOD of this solution is measured by determining the quantity of an electric current flowing between the working electrode (20) and counter-electrode (7) or reference electrode (5), applying a voltage between the electrodes by a potentiostat (6). By this method, the concentrations of organic compounds can be measured directly without using an oxygen electrode even when the quantity of dissolved oxygen is small.