A gate-power-supply device comprising: - a DC power supply; - an inverter circuit for converting DC power from the DC power supply into high-frequency power and for outputting the high-frequency power; - a transformer composed of a primary-side coil supplied with the high-frequency power, and a plurality of secondary-side coils wound around transformer cores through which the primary-side coil passes; - a plurality of rectifier circuits for respectively rectifying high-frequency powers outputted from the respective secondary-side coils of the transformer, the plurality of rectifier circuits being adapted for respectively driving gate drivers for a plurality of semiconductor switching devices, by outputs from the rectifier circuits; - a plurality of secondary-side parallel capacitors (C P) connected in parallel to the respective secondary-side coils, the plurality of secondary-side parallel capacitors (C P) being adapted for canceling inductance components of the secondary-side coils at drive frequency of the inverter circuit; - a primary-side series capacitor (C S) connected in series to the primary-side coil, the primary-side series capacitor (C S) being adapted for canceling an imaginary term (inductance component) of a combined impedance of the gate drivers, the rectifier circuits, the secondary-side parallel capacitors (C P), the secondary-side coils, the transformer cores, and the primary-side coil, which are a load viewed from the inverter circuit; and - a sequential power transfer section which is provided between the transformer and the plurality of rectifier circuits to sequentially transfer power supplied via the secondary-side coils of the transformer to the plurality of rectifier circuits, wherein the gate drivers for the plurality of semiconductor switching devices are respectively driven via the sequential power transfer section and the plurality of rectifier circuits, wherein the transformer cores are formed in an arc shape having an opening. - a DC power supply; - an inverter circuit for converting DC power from the DC power supply into high-frequency power and for outputting the high-frequency power; - a transformer composed of a primary-side coil supplied with the high-frequency power, and a plurality of secondary-side coils wound around transformer cores through which the primary-side coil passes; - a plurality of rectifier circuits for respectively rectifying high-frequency powers outputted from the respective secondary-side coils of the transformer, the plurality of rectifier circuits being adapted for respectively driving gate drivers for a plurality of semiconductor switching devices, by outputs from the rectifier circuits; - a plurality of secondary-side parallel capacitors (C P) connected in parallel to the respective secondary-side coils, the plurality of secondary-side parallel capacitors (C P) being adapted for canceling inductance components of the secondary-side coils at drive frequency of the inverter circuit; - a primary-side series capacitor (C S) connected in series to the primary-side coil, the primary-side series capacitor (C S) being adapted for canceling an imaginary term (inductance component) of a combined impedance of the gate drivers, the rectifier circuits, the secondary-side parallel capacitors (C P), the secondary-side coils, the transformer cores, and the primary-side coil, which are a load viewed from the inverter circuit; and - a sequential power transfer section which is provided between the transformer and the plurality of rectifier circuits to sequentially transfer power supplied via the secondary-side coils of the transformer to the plurality of rectifier circuits,