A light source device including a light-guiding pillar, an opaque layer, and a light source is provided. The light-guiding pillar has a light incident surface and a light emitting surface, and the light incident surface is adjacent to the light emitting surface. In addition, the opaque layer is disposed on the surfaces of the light-guiding pillar outside the light incident surface and the light emitting surface. The light source is disposed near the light incident surface. The light emitted from the light source propagates into the light-guiding pillar through the light incident surface, and then emits from the light emitting surface.