A largest value is selected from brightness data of sub-pixels of each pixel in an image, and a first light source luminance is calculated using the largest value. An average of the largest value of each pixel is calculated, and a second light source luminance is calculated using the average. By comparing the first light source luminance with the second light source luminance, an output light source luminance as a weighted average of the first light source luminance and the second light source luminance is calculated by setting a larger weight to a smaller one of the first light source luminance and the second light source luminance. A gradation of each sub-pixel is converted using the output light source luminance. A light source unit is controlled to emit the light having the output light source luminance.