The transparent light guide plate includes a rectangular light exit surface, a thick portion positioned at substantially a central portion of the surface, thin edge portions formed in parallel on both sides of the thick portion, a parallel groove accommodating a bar-like light source and being formed at substantially a center of the thick portion and inclined rear portions whose thickness is reduced from the thick portion toward the thin edge portions. An end portion of the groove is narrowed toward the surface symmetrically to a center line of the groove in a sectional shape thereof according to a ratio of a peak value of illuminance or luminance of emitted light at a first portion of the surface corresponding to the groove to an average value thereof at second portions corresponding to the inclined rear portions. The planar lighting device includes the light guide plate. The liquid crystal display device includes the planar lighting device.