A diffusion lens includes a concave first lens surface, a convex second lens surface such that part of light incident on the first lens surface is output from the second lens surface, a side surface extending from a periphery of the second lens surface in a vertical direction of the diffusion lens such that part of the rest of the light incident on the first lens surface is output from the side surface, and a bottom surface extending from a periphery of the first lens surface in a horizontal direction of the diffusion lens to meet a periphery of the side surface. Rates of change of the first and second lens surfaces have the same sign. A pattern is formed on the side surface and the bottom surface to diffuse the light through the side surface and diffuse the light reflected by the bottom surface.