To reduce an influence of external light to accurately detect the size of a document, in reading a document with thickness by a document reading device of a sheet-through type.

A scanner unit is configured to include a light source for thick paper at an upper part of a scanner, and a light-receiving unit constituted of a plurality of light-receiving elements at a lower part of the scanner; and a document conveyance path is constituted of a gap formed by the upper part and lower part of the scanner. When the constituted scanner unit reads a document with thickness, increases a distance of the gap formed by the upper part and lower part of the scanner to convey the document through the document conveyance path, and irradiates, of all the light-receiving elements constituting the light-receiving unit, a light-receiving element arranged at a position corresponding to an area outside the document, with white light from the light source for thick paper at the intensity in which the light-receiving element is saturated.