PURPOSE: To reduce the power consumption of a display device without making a user feel that the screen is dark or hard to see by providing a means which decreases the brightness of the display device stepwise at predetermined constant interval of time during the use of the device.

CONSTITUTION: When a CPU 1 sets a time for a timer controller(TC) 10 and indicates the start of measurement, the TC 10 controls a timer to measure the set time and sends an interruption signal to the CPU 1, which is informed of the time elapse. With this interruption signal, the CPU 1 puts a back light controller(BLC) 7 in operation. The BLC 7 controls the ON/OFF state and brightness of a back light fluorescent tube(BL) 6. The brightness control processing of this BL 6 is performed by varying, for example, an illumination voltage and performing control for repeating the illumination and extinction faster than the human eyes feel, adjusting the time rate of illumination and extinction, and using a filter which is variable in transmissivity. Thus, the back light source is made darker nearly as fast as the eyes follow up by darkness adaption.