To make the relation between luminance data and light emission luminance of LEDs(light emitting diodes) match with the gamma characteristics of a CRT (cathode-ray tube) without increasing the number of bits of the luminance data and without applying the pre-processing such as correction to the luminance data.

Luminance data Sv converted into binary codes in an A/D converter 4 are converted into serial data in a control part 3 to be outputted to respective pulse width modulation circuits 1 which are cascaded. Then, pulse currents having pulse lengths corresponding to the luminance data Sv are generated in respective pulse width modulation circuits 1 and LED 2 connected to the circuits 1 are driven with these currents to emit light. Moreover, amplitudes of these currents are varied in accordance with count values of counters counting the cycles of the pulse currents with a clock signal. Thus, the relation between time-wise means values of the pulse currents flowing through the LED 2 and the luminance data matches with the gamma characteristics of the CRT.