A luminance control apparatus for a backlight of a display device in a vehicle has a first transistor and a second transistor connected to the backlight. The first transistor is controlled by pulse-width modulation control, and the second transistor is controlled to supply a low level current at nighttime and a high level current at daytime under a condition that the first transistor is ON. The duty ratio of the first transistor is varied over a full range from 0% to 100% at the nighttime, while the duty ratio of the same is varied only in a partial range at the daytime.