PURPOSE: To stationarily observe the high speed change status of an LSI etc coated with a passivation coat by providing a gate circuit synchronizing with the phase change of a phase circuit that varies the phase change of a pulse which makes chopping of an electron beam and allowing a picture signal to pass through said gate circuit only for a certain phase change period so as to have an image displayed.

CONSTITUTION: Only when a switch 14 that switches the irradiatation timing of a pulse and a beam is ON, a resistor 15 is connected and the voltage on a passivation coat is detected as current 16. The voltage of the waveform shown in the figure is applied to an LSI electrode 18 and the switch 14 is set to ON only in the phase of wave form an LSI electrode 18 and the switch 14 is set to ON only in the phase of wave form shawa by the arrowheads. For the voltage of an electrode 19 (measurement point), a signal is not detected gradually by the actuation of a capacitor 13. To avoid this, the timing (phase) for setting the switch 14 to ON is moved finely four times each when the voltage is applied and when the potential is zero. If the cycle that moves this phase becomes shorter than time constant, this attenuation can mostly be ignored.