analog operation unit 46 shown in fig8 may be modified into an analog circuit shown in fig1 next the real time measurement of differential conductance values at a specified bias voltage and the distribution image measurement are explained a circuit of fig1 is a sampling circuit used for always outputting the differential conductance value at the specified voltage a differential conductance value u2202 i t u2202 v t output from a differential conductance operation circuit shown in fig1 is input to a sample hold amplifier 70 a bias voltage v t and a reference voltage vo are input to a comparator 71 which in turn outputs a corresponding binary signal and a shot pulse generator 72 for generating a pulse whose pulse width u03c4 p generates a pulse which is sampled by the sample hold amplifier 70 at a timing of v t vo thus the differential conductance value u2202 i t u2202 v t v t v o set when v t vo can be always output if the scanning speed of the probe is set such that the servo operation can follow the surface irregularity a sample probe distance is kept constant and the surface irregularity information and the differential conductance value at the bias voltage vo are displayed in correspondence with the xy scanning signals then the spatial relation between the unevenness image and differential conductance image can be displayed and compared on the real time basis since the differential conductance can be independently sampled at a large number of reference voltages if sampling circuits are connected in parallel values of the local differential conductance at a plurality of bias voltage levels can be simultaneously held that is a large number of images can be displayed at the same time fig1 a and 19b.