When there is a temperature difference along the length of a cold cathode tube used as a light source for a display device, a cataphoresis phenomenon occurs due to movement of mercury enclosed contained within the tube, thereby shortening the tube life. In order to solve this problem, temperature sensors are provided to detect a temperature difference between both the ends of the tube. A duty cycle of a voltage waveform to be applied from a DC/AC inverter to the tube is changed so as to cancel movement of the mercury due to the temperature difference.