PURPOSE: To save electric power to be consumed by luminous elements as well as improve the visibility of a displayer by a method in which the brightness of the outside is detected and the emission amount of the luminous elements of the displayer is controlled in such a way as to increase it in proportion with the brightness of the outside.

CONSTITUTION: According to the brightness of the outside, the brightness of a lamp 7 for high-boom display provided in an indicator is controlled. When the outisde is bright, light coming into a photo transistor 16 is intensified, output of a comparator 19 is lowered to a low level, transistors 21 and 9 are turned ON, the transitor 9 becomes a connected state, and the lamp 7 becomes brighter. When the outside is dark, light coming into the transistor 16 is weakened, output of the comparator 19 is raised to high level, the transistor 9 is held in a disconnected state, current flows in a resistor 8 in a limited state, and the lamp 7 becomes darker.