An image reading apparatus includes: a reading table having a transparent plate having an exposed area which constitutes a part of an outer surface of the table exposed to the exterior, and on which a document with an image thereon is placed; a document holding member movable relative to the table and between a closed and an open position, and thus holding the document and covering the plate; a scanning device disposed on a side of the plate opposite to the side on which the document is placed on the plate, the scanning device being reciprocated in a direction along the plate; an image reading device mounted in the scanning device and including a light source, the reading device emitting light from the light source toward the document on the plate, and receiving the reflected light from the document, to read the image thereby; a control unit controlling reading by the reading device, and a reciprocating movement of the scanning device; a reference member disposed within the exposed area, and used as a reference when an amount of the light emitted from the light source is adjusted; and an external-light detector determining whether external light is entering the table, based on at least one value obtained as a result of reading the reference member.