Abstract: A document size sensing device incorporated in an image recorder and capable of sensing a plurality of document sizes with a small number of optical elements. A sensor is made up of a single light emitting element, a spectroscope for dividing light issuing from the light emitting element into a plurality of beams toward a plane where a document is laid, a lens on which light reflected by the document is incident, and a plurality of light-sensitive elements for receiving the light propagating through the lens. A single sensor with such a configuration can sense a plurality of document sizes.