PURPOSE: To execute safely and with a high accuracy a measurement without generating a spark discharge by forming a pair of upper and lower auxiliary electrodes of a capsule type, containing an electric field sensor with both these auxiliary electrodes, and sealing up tightly.

CONSTITUTION: An electric field sensor 16' is constituted so that an intermediate electro-optical crystal 13 is formed to a size which has formed a projecting part 13' of a necessary size on one side of its outside, and a light from a light source 31 provided separately is irradiated to the projecting part 13' through an optical fiber cable 32. Electro-optical crystals 11, 12, 13 and 14 used for the electric field sensor 16' are formed by using that which has both a photoconductive effect and an electro-optical effect, for instance, Bi12SiO20. When static electricity of oil in an oil layer is detected by using the electric field sensor 16', an exact value can be known without being influenced by a varied portion (drift) of Po. A cylindrical body 42 passing up and down through a float 41 consisting of an insulator is formed as one body, and fixed to an enclosed state by screwing and clamping an upper auxiliary electrode 43 and a lower auxiliary electrode 44 of a capsule type to an external thread formed on the upper and lower outsides of this cylindrical body 42.