PURPOSE: To measure voids independent of a void border material by constituting a thin 3-terminal capacitive transducer set in a gap to be measured and measuring the void between two borders utilizing capacitance.

CONSTITUTION: The capacitive sensor or a transducer 18 comprises a pair of electrodes 12 and 14 assigned with an interval, and a combination plate 20 is assigned behind, while apart from, the electrodes 12 and 14 in the plane parallel to the electrode. A member 2 comprising an insulating dielectric material is assigned between the combination plate 20 and the electrodes 12 and 14, so that air space 24 and 26 and a central groove 28 are formed, with the layer of an insulating dielectric material 30 attached to the combination plate 20. An earth plate 32 is assigned below an insulation/combination structure, being the reference of physical contact against one side of void to be measured, or a border. Then, by communication through coaxial carries 34 and 36, the distance between two borders is measured.