PURPOSE: To remove a very small amount of solid substance, ion, hydrolyzable gas, etc., in an inert liquid by bringing a fluorine-based inert liquid to be used as an insulating refrigerant for gas insulating electric machine into contact with water and then dehydrating.

CONSTITUTION: A contaminated fluorine-based inert liquid is put in a container, water is poured to the container from a water sprinkling pipe introduced to the bottom at about 200ml/minute flow rate and overflown for 1 hour. Water at the upper part is removed as much as possible by decantation and the fluorine-based inert liquid is passed through dried filter paper to remove residual water to give the purified liquid. By the method, improvement in electrical characteristics such as intrinsic resistance, dielectric dissipation factor and breakdown voltage can be attained.