The present invention relates to a fryer provided with a system that generates a high-density electromagnetic field, preventing deterioration of cooking oil. The fryer comprises: an oil container holding cooking oil; a high-voltage power supply to generate high voltage; and an electrode unit connected to the high-voltage power supply. This electrode unit comprises an electrode made of a screw rod. Because the spiral thread has tips in a shape of acute angle, electric fields tend to concentrate around the acute-angle edges on the entire surface, and thus a high-density electromagnetic field is generated in the oil container.