PURPOSE: To obtain the subject fluid exhibiting dispersion stability, rust- preventing effect and anti-oxidation effect without lowering the electro-viscous effect and electrical insulation by dispersing dielectric particles having specific particle size in an electrically insulating liquid and adding a specific amount of precipitating fine particles.

CONSTITUTION: The average particle diameter of the main component particles of dielectric particles is 3-100μm (preferably 5-50μm) and the average particle diameter of the dielectric particles is ≤0.3μm and ≤20% of that of the main component particles. The content of fine particles precipitating in an electrically insulating liquid is 1-20vol.% (preferably 2-10vol.%) based on the whole particles.