PURPOSE: To prevent a display from becoming too dark to see or being so bright that the display contents are glaring by controlling the luminance of display light corresponding to adaption characteristics of the human visual sensation to lightness when external light varies in lightness.

CONSTITUTION: The lightness signal of the external light detected by CdS 20 in time series is amplified by an operational amplifier 22 and converted by an AD converter 24 into a digital signal, and a central processing unit 23 calculates variation in the lightness in time series according to the lightness of the external light. Further, the brightness set value of the display light outputted from an output port 25 to a triac control circuit 16 is fed back to the central processing unit 23, which calculates deviation between the lightness of the external light and the brightness set value of the display light in time series. A chip 28 for fuzzy arithmetic calculates the quantity of variation in the brightness set value of the display light in time series from the lightness variation and the deviation. The central processing unit 23 fetches the brightness set variation quantity, calculates a brightness set value at a next point of time in time series, and controls the triac control circuit 16 according to the set value to vary the brightness of a back light 6.