An image reading apparatus includes a reading controller, a data generating circuit unit, a document holder, an opening/closing sensor which provides an output of a first level when the document holder is open by an angle equal to or more than a predetermined angle, and a storage unit which stores an adjustment value. Where a return is made from a power saving mode to an active mode, when an output level of the opening/closing sensor is the first level, the reading controller makes a setting for an operation of the data generating circuit unit based on the adjustment value stored in the storage unit, and then makes the reading unit perform reading for document size detection.