Provided is a method for accurately quantifying a chemical substance contained in a sample solution at a significantly low concentration of not more than 1×10−8M. First, prepared is a measurement system including a counter electrode 13, a first reference electrode 12, a first working electrode 11a, a second working electrode 11b and a second reference electrode 14. Second, voltages of V1 volts and V2 volts (V1>V2) are applied to the first working electrode 11a and the second working electrode 11b, respectively. Third, a voltage difference ΔE between the second working electrode 11b and the second reference electrode 14 is measured. Finally, the concentration of the chemical substance is calculated on the basis of the voltage difference ΔE.