An image reading device includes a red light source, a green light source, a blue light source, a white reference plate, a light-amount adjusting section, a photo-detecting section, and a signal processing section. The light-amount adjusting section sets lighting periods of the red, green, and blue light sources for reading in a monochrome mode, based on: lighting periods of the red, green, and blue light sources set for reading in a color mode; and a result of detection performed by the photo-detecting section when the white reference plate is illuminated with the red light, the green light, and the blue light respectively emitted by the red, green, and blue light sources for the respective lighting periods set for reading in the color mode.