In a cold-cathode dimmer, a current detector includes a LPF for removing the frequency component of a lighting/driving signal for a lighting driver from the output of a diode for rectifying the voltage across a current detecting resistor and passing the frequency component of an intermittent control signal for controlling the duty ratio of the lighting/driving signal. A luminance controller includes a current value computing section for producing, as a current average value, a multiplied result of a ratio of an interval during which the output level of the LPF is higher than a prescribed level to a period of the output of the LPF and the output level of the LPF during the interval during which it is higher than the prescribed level, and a duty controller for stabilizing a luminance of cold-cathode tubes at a target luminance in such a manner that the duty ratio of the intermittent control signal is set at a value corresponding to a difference between the current average value outputted from the current value computing section and a target value thereof.