An image reading device includes a platen glass, a document feeder, a light source, a light receiving section, a first opening/closing detection sensor, a second opening/closing detection sensor, and a document size determination section. The document size determination section determines a size of an original document on the basis of a measurement result from the light receiving section. Where the first opening/closing detection sensor indicates the open state, while the second opening/closing detection sensor indicates no open state when the original document is placed on the platen glass, the document size determination section determines a size of the original document in a main scanning direction on the basis of full lighting-up determination data obtained with all of plural light emitting elements lighted up and partial lighting-up determination data obtained with some of the plural light emitting elements lighted up.