Abstract: The present invention provides a high-frequency dielectric heating device comprises: a microwave output unit including an inverter unit using a semiconductor switching element, heat-radiating fins for radiating the heat generated by an IGBT, a printed board having a thermistor for detecting the temperature of the semiconductor switching element, wherein the thermistor being soldered to a leg portion of the semiconductor switching element or near to the leg portion thereof on the side of the soldering surface of the printed board, a booster transformer, a high-voltage rectifier unit, and a magnetron; and a heat-cooking chamber fed with microwaves radiated from the magnetron. When the thermistor has assumed a predetermined resistance, a power-down control operation is effected by greatly decreasing the power fed to the semiconductor switching element. Then, permitting the power fed to the semiconductor switching element to vary depending upon the resistance of the thermistor.