A directional light distributing optical element includes a light incident surface and a light emission curved surface. The light incident surface receives a light emitted by a light source. The light emission curved surface and a first plane are intersected to form a first curve. The first curve has a plurality of first curve segments, and each first curve segment includes at least three first tangent points. After passing each first tangent point along a connecting line of the light source and each first tangent point, the light exits along a first axis, and an included angle formed between the first axis and an optic axis is greater than −15° and smaller than 15°. Thus, the light after passing the directional light distributing optical element forms a one-dimensional directional light.