A document reading apparatus includes a document positioning plate configured to set a document to be read thereon, a light source configured to emit light onto an area where the document is set on the document positioning plate, an image sensor configured to photoelectrically convert reflected light of the light emitted from the light source to output color signals, and a document size detection unit configured to select a color signal used for document size detection from among the color signals output from the image sensor based on the color signals output from the image sensor in a state where the light source is turned off, and to detect a size of the document set on the document positioning plate based on the selected color signal output from the image sensor in a state where the light source is turned on.