Constant temperature regulation of a hybrid heater which is composed of a temperature sensitive ferrite with low permeability and a Cu tube is investigated for hyperthermia. The temperature sensitive ferrite is inserted into a Cu tube and its length and diameter are 10 mm and 3 mm. Below B=0.05 T, the measured temperature and the calculated one increased with the ratio B^1/2of and agreed well with each other. Above B=0.05 T, the measured temperatures maintained constantly almost 50℃±1.5℃because of the influence of Curie temperature of the temperature sensitive ferrite. This result shows that the hybrid heater is able to regulate the temperature constantly at the rate of 50℃±1.5℃.