PURPOSE: To enable potential to be measured with high precision through a passivation layer.

CONSTITUTION: The rectangular waveform of voltage is repeatedly applied from a function generator 15 to a cantilever 12 having a probe 11. The wiring pattern 17 of an electronic device 13 as a measurement object is kept at certain potential via a variable power supply 14. The probe 11 and cantilever 12 vibrate, due to Van der Waals force and an electrostatic force acting between the probe 11 and the device 13. The amplitude of the vibration is detected with a semiconductor laser device 31, a reflector 32 and a two-division photo-detector 33, and amplified with an amplifier 34. Thereafter, the detected amplitude is processed and converted to data with a processing device 35, thereby calculating potential at the predetermined position. Then, the calculated potential is shown on a CRT display 36. An inter-atomic force microscope(AMF) or an optical microscope may be used to show the surface contour of a measurement position on another CRT display 37.