An illuminating device includes: a light guide plate for outputting light, which enters the light guide plate from one side surface, from a front surface of the light guide plate; an optical sheet disposed on the front surface of the light guide plate; and a reflection sheet disposed on a rear surface of the light guide plate. The optical sheet includes prism arrays which are provided on a surface of the optical sheet opposite to the light guide plate, and each of which has at least two inclined surfaces and a ridge line extending in one direction. The light guide plate changes a polarization state of light that is reflected by the surface of the optical sheet, is transmitted through the light guide plate, is reflected by the reflection sheet, is transmitted through the light guide plate again, and then enters the optical sheet.