A small-size and light-weight backlight system and a planar display device has a structure such that luminance unevenness and chromaticity unevenness in the vicinity of the incident light portion of a light guide plate are not noticeable. The back light system includes a light guide plate arranged to guide the light from a light source and emit the light in a planar direction. The light source includes a linear light source body arranged in the vicinity of the side edge of the light guide plate. The linear light source body includes a linear body in which a plurality of light emitting elements are embedded in the longitudinal direction. A light direction change sheet is arranged on the light emission surface of the light guide plate. The light direction change sheet has a plurality of protruding stripes arranged substantially parallel to the longitudinal direction of the linear light source body formed on the surface opposing to the light guide plate.