A water purifying unit generates pure water by eliminating impurity ions from dielectric fluid. A corrosion-protection-ion generating unit generates corrosion protection ions, substitutes predetermined cations for impurity cations contained in the dielectric fluid, and substitutes the corrosion protection ions for impurity anions contained in the dielectric fluid. A switching unit switches a supply of the dielectric fluid between the water purifying unit and the corrosion-protection-ion generating unit. A switching control unit controls the switching unit based on a conductivity of the dielectric fluid measured by a conductivity measuring unit.