An error amplification circuit amplifies an error voltage between a reference voltage and an output voltage of the LED current sensor. A control circuit controls the DC output power so that the DC output power becomes a predetermined value, by performing on-off control on a switching element based on the output voltage of the error amplification circuit. The error amplification circuit includes an error amplifier, a phase compensation circuit connected between an inverting input terminal and an output terminal of the error amplifier and having a serial circuit of a phase compensation capacitor and a phase compensation resistor, a gain adjustment resistor serially connected at one end to the serial circuit and connected at the other end to the LED current sensor, and a short circuit configured to make the inverting input terminal of the error amplifier grounded for a predetermined period of time after activation.