In first mode, controller is configured, when receiving a dimming instruction signal, to control voltage-adjusting part to provide a DC voltage with a magnitude corresponding to a luminous level, across output ends of DC power supply, and apply a control voltage with a magnitude kept constant to a control terminal of switching element. In second mode, controller is configured, when receiving the dimming instruction signal, to apply the control voltage with a magnitude corresponding to luminous level to the control terminal of switching element, and control voltage-adjusting part to provide the DC voltage with a magnitude kept constant, across the output ends of DC power supply. Controller is configured to operate in one of first and second modes when luminous level is predetermined level or more, and in the other of first and second modes when luminous level is less than predetermined level.