PURPOSE: To enhance the speed and sensitivity of test by supplying a sample voltage from a drive unit in place of a regulation loop for limiting the processing speed thereby tracking the reverse voltage to obtain a maximum measurement sensitivity. CONSTITUTION: A set voltage UE1 is fed from a drive unit AE to the terminal A1 of a sample P and an actual sample voltage stroke UP1 is generated at a measuring position M. A reverse voltage UG is superposed on a basic reverse voltage UG1 generated by a unit AE to generate a target sample voltage stroke UPS and a reverse voltage UG is formed at a reverse field electrode GN along with the reverse voltage UG1 in order to track the reverse voltage UG. When the set voltage UE1 is varied, a detection current IA is settled and since the detection current IA matches a target detector current I1 so long as the stroke UP1 at the measuring position M has a value equal to that of the stroke UPS, error of the sample P can be detected based on the variation of the detection current IA.
