A variable-frequency oscillator 1, a semiconductor power amplifier 2 for amplifying the output of the variable-frequency oscillator 1; a radiator 3 for radiating an electromagnetic wave for heating based on the output of the semiconductor power amplifier 2; a reflected wave monitoring circuit 5 for detecting a reflected wave of the electromagnetic wave for heating; and a controller 7 for controlling the oscillation frequency of the variable-frequency oscillator 1 are provided. The controller 7 changes the oscillation frequencies of the variable-frequency oscillator 1 discontinuously, thereby getting a frequency-hopping spread-spectrum radiation done by the radiator 3. The electromagnetic wave radiated by the radiator 3 irradiates an object 9 to be heated (which is usually food) inside a heating chamber 8, thereby heating the object.