In order to provide a larger-sized light guide plate that is thin and lightweight and allows more uniform, less uneven and higher-luminance illuminating light to emit from a light exit surface, a sectional shape of each back surface of the rectangular light exit surface satisfies the following 10th-order function formula. X is a position from the origin in a direction parallel to the one side of the light guide plate, and Y is a position from the origin in a direction perpendicular to the one side. Y=A0+A1×X+A2×X2+A3×X3+A4×X4+A5×X5+A6×X6+A7×X7+A8×X8+A9×X9+A10×X10, −1.3×105≦A0≦2.4×105, −2.0×104≦A1≦9.4×104, −2.6×104≦A2≦5.7×104, −5.5×103≦A3≦2.2×103, −2.1×102≦A4≦1.1×102, −3.7×10≦A5≦2.2×10, −1.4×10≦A6≦3.3×10, −3.6≦A7≦1.5, −7.8×10−2≦A8≦3.3×10−2, −1.2×10−2≦A9≦3.2×10−2, −1.4×10−3≦A10≦4.9×10−4. A planar lighting device includes the light guide plate, and a liquid crystal display apparatus includes the planar lighting device as a backlight unit.