In the case of the I/V curve consisting of an almost rectilinear line which includes small dI/dV and d2I/dV2 term, a lock-in amplifier is generally used for the detection and measurement of these derivatives. The digital technique of using an A/D converter and the numerical analysis are powerless for the detection and measurement of small derivatives, because these small derivatives are buried under the least significant digit voltage. In this research, we propose a new method to detect and measure these small derivatives employing an analog subtraction circuit and an amplifier for the pretreatment of A/D conversion. This method effectuates the fast and simple scanning tunneling spectroscopy.