PURPOSE: To adsorb and remove efficiently water from oil in a short time by immersing a water-absorptive filter material consisting of a fibrous high- performance water-absorptive resin and an inorg. adsorbent in a moisture- absorptive refrigerator oil and bringing the material into contact with the oil while giving a dynamic vibration to the material.

CONSTITUTION: A water-absorptive filter material 1 is prepd. by sandwiching an inorg. adsorbent layer 3 (e.g. a silica gal layer) between the layers of a fibrous high-performance water-absorptive resin 2 of a synthetic fiber, a semi- synthetic fiber or a natural fiber type. Then, this material 1 is immersed in a moisture-absorptive refrigerator oil and brought into contact with the oil while giving a dynamic vibration to the material to adsorb and remove water from the refrigerator oil. As the water-absorptive resin 2 with a large water- absorbing capacity and the inorg. adsorbent layer 3 with a large water-absorbing speed are combined, it is possible to adsorb and remove efficiently water from the refrigerator oil. Therefore, it is possible to prevent readily a failure in cooling caused by water in the refrigerator oil and acceleration of deterioration of the oil.