PURPOSE: To measure the voltage of an electrode on a circuit board, to be measured, stably without resonance even at a high frequency in an apparatus wherein an electro-optical element whose optical characteristic is changed when an electric field is applied is arranged close to the circuit board and a change in the optical characteristic is measured by an optical beam.

CONSTITUTION: An auxiliary electrode 5 is installed on the surface of an electro- optical element 1, and it is connected to a reference electrode 4 on a circuit board 2, to be measured, via conductive enclosures 6, 8 installed so as to surround the electro-optical element 1. Consequently, since a voltage across an electrode as a measuring object and the reference electrode 4 is applied effectively to the electro-optical element 1, the high sensitivity of the title detection apparatus can be obtained, and the voltage of the electrode with reference to a reference voltage can be measured irrespective of the geometrical arrangement of the electrode. In addition, since a resonant frequency can be made large, a measurable frequency band can be expanded. In addition, the influence of an external electric field can be eliminated.