Provided is an optical sensor mounting structure which is used in an image display device and in which the gap between a reflection sheet and a tubular cushion for preventing the entry of external light into an optical sensor is eliminated so that the amount of light from a backlight can be measured accurately. A liquid crystal image display device 1 includes an optical sensor 12 that measures light from the back surface of a reflection sheet 104, a substrate 11 having the optical sensor 12 thereon, and a tubular cushion 13 for preventing the entry of external light into the optical sensor 12. The front surface of the tubular cushion 13 is bonded to the reflection sheet 104, and the back surface thereof is bonded to the substrate 11.