A gate-power-supply device is provided with an inverter circuit, a transformer, and rectifier circuits. The device includes secondary-side parallel capacitors, connected in parallel to secondary-side coils of the transformer, for cancelling inductance components of the secondary-side coils at the drive frequency of the inverter circuit. The device includes a primary-side series capacitor, connected in series to a primary-side coil of the transformer, for cancelling the imaginary term (inductance component) of the combined impedance of the gate drivers, the rectifier circuits, the secondary-side parallel capacitors, the secondary-side coils, transformer cores and the primary-side coil, which are a load viewed from the inverter circuit.