An image reading apparatus includes a reading condition receiving unit for receiving a reading condition; an image reading unit for reading the original according to the reading condition to obtain image information; a storage unit for storing the image information; a reading mode selection unit for selecting a first reading mode or a second reading mode according to a result of comparison between an image information amount and a memory capacity of the storage unit; and a data reading unit for reading the image information from the storage unit according to the reading mode. The data reading unit reads the image information per page in the first reading mode. The data reading unit reads the image information per block in the second reading mode.