A new method of plotting the impurity profile is described. It makes efficient use of the advantage of an operational amplifier and the nonlinearity of the junction capacitance in a semiconductor. When two small ac signals are applied to the inverting input terminal of the operational amplifier, intermodulation arises due to the nonlinearity of the junction capacitance inserted in the feedback loop, at the time the reverse bias voltage to the junction can be supplied from the noninverting input terminal; then the signals concerning both the depletion layer widthxand the impurity concentrationN(x) arise at the output terminal of the operational amplifier. By detecting each of the amplified input signals and the difference‐frequency ones corresponding toxandN(x), respectively, the relation ofx−N(x) can be plotted easily, at the same time, in a short time, and at a low cost.