A line of sight of a driver is detected based on an image of the driver obtained by image-capturing; whether the driver is viewing the image display unit or viewing an outside of the image display unit is judged based on the detected line of sight; glare is detected based on the image of the driver; and a luminance level of the image displayed by the image display unit is determined based on the glare at the time of viewing the image display unit and the glare at the time of viewing the outside of the image display unit. The visibility of the displayed image can be improved even when the driver feels glare because of external light falling upon the driver's eyes.