A detection device is provided with a light-receiving module configured to output an electrical signal according to a power of an input light received at a light-receiving surface, an optical lens configured to collimate and guide to the light-receiving surface the input light from the outside, and at least one condensing unit that is provided in a path of the input light between the optical lens and the light-receiving surface and configured to decrease a beam diameter of the input light at the light-receiving surface.