The microwave signal having passed the band-pass filter (
406) may be inputted into the magnetron (
402). The microwave signal filtered by the band-pass filter (
406), i.e., the microwave signal generated by the oscillation of the magnetron (
402) may be fed-back to the magnetron (
402), whereby the oscillation of the magnetron (
402) may be synchronized by the fed-back microwave signal. The microwave signal outputted by oscillation of the magnetron (
402) and the fed-back microwave signal may be combined to cause the magnetron (
402) to generate a microwave signal having a fixed frequency.