A resonant power conversion device comprising: a main circuit provided with a semiconductor switching element having a high-potential terminal, a low-potential terminal and a control terminal, and provided with a capacitor and an inductor connected in series or parallel to the high potential terminal or the low potential terminal, and the main circuit being configured to convert and output input power; and a drive circuit configured to output a control signal (Vgs) to the control terminal and drive the semiconductor switching element at a required switching frequency, wherein the semiconductor switching element is configured to enter an off-state or an on-state depending on a signal value of the control signal (Vgs) input to the control terminal, characterized in that: the output capacitance of the semiconductor switching element behaves like a variable capacitance in its off-state according to the signal value of said control signal (Vgs), the drive circuit is configured to control the resonant frequency in the main circuit by changing the signal value of the control signal (Vgs) at which the semiconductor switching element enters the off-state, using two or more control signal values (V0, V1, V2) for the off-state, wherein the drive circuit is configured, when changing the semiconductor switching element to a second off-period coming after an on-period after a first off-period, to change a signal value of the control signal (Vgs) in the second off-period from a signal value of the control signal (Vgs) in the first off-period in order to harmonize the resonant frequency generated in the main circuit with the switching frequency, so as to reduce switching loss. a main circuit provided with a semiconductor switching element having a high-potential terminal, a low-potential terminal and a control terminal, and provided with a capacitor and an inductor connected in series or parallel to the high potential terminal or the low potential terminal, and the main circuit being configured to convert and output input power; and a drive circuit configured to output a control signal (Vgs) to the control terminal and drive the semiconductor switching element at a required switching frequency, wherein the semiconductor switching element is configured to enter an off-state or an on-state depending on a signal value of the control signal (Vgs) input to the control terminal, characterized in that: the output capacitance of the semiconductor switching element behaves like a variable capacitance in its off-state according to the signal value of said control signal (Vgs), the drive circuit is configured to control the resonant frequency in the main circuit by changing the signal value of the control signal (Vgs) at which the semiconductor switching element enters the off-state, using two or more control signal values (V0, V1, V2) for the off-state, wherein the drive circuit is configured, when changing the semiconductor switching element to a second off-period coming after an on-period after a first off-period, to change a signal value of the control signal (Vgs) in the second off-period from a signal value of the control signal (Vgs) in the first off-period in order to harmonize the resonant frequency generated in the main circuit with the switching frequency, so as to reduce switching loss. the output capacitance of the semiconductor switching element behaves like a variable capacitance in its off-state according to the signal value of said control signal (Vgs), the drive circuit is configured to control the resonant frequency in the main circuit by changing the signal value of the control signal (Vgs) at which the semiconductor switching element enters the off-state, using two or more control signal values (V0, V1, V2) for the off-state, wherein the drive circuit is configured, when changing the semiconductor switching element to a second off-period coming after an on-period after a first off-period, to change a signal value of the control signal (Vgs) in the second off-period from a signal value of the control signal (Vgs) in the first off-period in order to harmonize the resonant frequency generated in the main circuit with the switching frequency, so as to reduce switching loss.