PURPOSE: To reduce the power consumption or to realize a long service life of a back light by detecting the brightness of the outside with a photo-sensor, and regulating the brightness of the back light responding to the resultant luminous intensity.

CONSTITUTION: A visual light sensor 1 detects the brightness of the outside, and outputs the electric energy responding to the luminous intensity. A brightness controller 2 controls the brightness of a back light 4 responding to the output of the sensor 1. A driving unit 3 drives the back light 4 in a driving condition responding to the output of the brightness controller 2. In such a way, by controlling the brightness adequately automatically responding to the circumstances, to suppress the brightness of the back light automatically in a bright place, and to increase the brightness in a dark place, the reduction of the power consumption is realized.