A high frequency heating apparatus for driving a magnetron, comprising: a DC power supply which is constituted by an AC power supply, a rectifying circuit for rectifying an AC voltage of said AC power supply, and a smoothing capacitor for smoothing an output voltage of said rectifying circuit; a series circuit constituted by two pieces of semiconductor switching elements (6, 7); a resonant circuit in which a primary winding of a leakage transformer and a capacitor are connected, said series circuit being connected parallel to said DC power supply, and one end of said resonant circuit being connected to a center point of said series circuit and the other end of said resonant circuit being connected to one end of said DC power supply in an AC equivalent circuit; driving means for driving said semiconductor switching elements respectively; rectifying means connected to a secondary winding of said leakage transformer; and the magnetron connected to said rectifying means; characterized by a control signal forming circuit for forming a control signal from a difference between an input current of the AC power supply and a reference current, or a difference between a voltage to be applied to the magnetron and a reference voltage; a frequency modulated signal forming circuit for correcting rectified voltage/rectified current which are obtained by rectifying the AC voltage/current of the AC power supply based upon the control signal of the control signal forming circuit; a dead time forming circuit for receiving the output of the frequency modulated signal forming circuit; and a control signal converting circuit having a function capable of further lowering the value of said control signal, disposed between said control signal forming circuit and said frequency modulated signal forming circuit. a DC power supply which is constituted by an AC power supply, a rectifying circuit for rectifying an AC voltage of said AC power supply, and a smoothing capacitor for smoothing an output voltage of said rectifying circuit; a series circuit constituted by two pieces of semiconductor switching elements (6, 7); a resonant circuit in which a primary winding of a leakage transformer and a capacitor are connected, said series circuit being connected parallel to said DC power supply, and one end of said resonant circuit being connected to a center point of said series circuit and the other end of said resonant circuit being connected to one end of said DC power supply in an AC equivalent circuit; driving means for driving said semiconductor switching elements respectively; rectifying means connected to a secondary winding of said leakage transformer; and the magnetron connected to said rectifying means; characterized by a control signal forming circuit for forming a control signal from a difference between an input current of the AC power supply and a reference current, or a difference between a voltage to be applied to the magnetron and a reference voltage; a frequency modulated signal forming circuit for correcting rectified voltage/rectified current which are obtained by rectifying the AC voltage/current of the AC power supply based upon the control signal of the control signal forming circuit; a dead time forming circuit for receiving the output of the frequency modulated signal forming circuit; and a control signal converting circuit having a function capable of further lowering the value of said control signal, disposed between said control signal forming circuit and said frequency modulated signal forming circuit.