PROBLEM TO BE SOLVED: To readily stabilize phase of the oscillation output of a magnetron and make it controllable.
SOLUTION: By a high-voltage DC stabilizing power-supply 12, an anode current flows in the magnetron 11 to make it into oscillation state. By a circulator 16, a reference signal is fed into the magnetron 11 for synchronizing with the oscillation frequency. Microwaves, radiated from the magnetron 11 are extracted by the circulator 16, are allowed to be partially branched by a directional coupler 18, and are compared with the reference signal by a mixer 17. Then, the comparison result is inputted to an anode current control computing element 22, the frequency/phase difference between microwave output and the reference signal is obtained, and the anode current is controlled via the stabilization power-supply 12 for eliminating the error amount. By feedback control with respect to the anode current, the oscillation output of the magnetron 11 is locked very stably to the frequency and to the phase of the reference signal. Therefore, by changing the phase of the reference signal using a variable phase- shifter 14, the microwave output phase can be made readily variable.