A scanner identifies a document size on the basis of the intensity of light outputted in a photometric condition different from a first photometric condition, by photometric units provided to a position subject to determination that is a position excluding a position where light of an intensity not less than a first threshold value is received in the first photometric condition, among a plurality of positions to which the photometric units are provided. The first photometric condition is a condition where a document cover is open and a light-emitting unit is not emitting light.