A fluororesin-coated electric wire cable is wound around a ring-shaped groove defined by an outer periphery of a cylinder and two flange edges immersed in an edible oil in a tank, forming each oscillating unit having a coil part; and the oscillating units are connected to an electromagnetic wave generator through a relay unit. Electromagnetic waves are irradiated from the coil parts based on an alternating current having a single frequency, an alternating current having a plurality of single frequencies different from one another, or an alternating current having a timewise varying frequency, the alternating currents each being within a frequency band of 4 kHz to 25 kHz, and a treatment based on a far-infrared heater or far-infrared ceramic is used combinedly therewith, to prevent deterioration of an edible oil or industrial oil.