Provided is a high-frequency power supply device capable of causing an appropriate current to flow through a transformer. A self-oscillation high-frequency power supply device is provided with a DC power supply, an LC resonant circuit, a switching circuit, and a transformer. The LC resonant circuit includes an induction coil for plasma generation and a capacitor. The switching circuit includes a semiconductor element, the switching circuit being configured to subject DC power supplied from the DC power supply to switching processing to supply high-frequency power to the LC resonant circuit. The transformer includes a primary coil included in the LC resonant circuit and a secondary coil connected to the semiconductor element to turn on/off a semiconductor element. The transformer has a coaxial structure in which the primary coil and the secondary coil are coaxially provided. The LC resonant circuit includes a resistor connected in parallel to the primary coil.