An illuminating device includes light emitting elements extending in a lengthwise direction and a light guide plate having a first principal surface for emitting light from the light emitting elements and a second principal surface facing the first principal surface. The light guide plate has light guide regions corresponding to the light emitting elements. The light emitting elements overlap the light guide plate on a side of the second principal surface of the light guide plate in plan view. The side of the second principal surface of the light guide plate has first sectional surfaces, on which light originated from each of the light emitting elements is incident. The side of the second principal surface of the light guide plate has second sectional surfaces, each having a reflection surface for reflecting the light originated from each light emitting element toward the first sectional surface, corresponding to the light emitting elements.