An image reader is provided, which includes a controller that when a PC-scan mode is set as an output mode, the controller controls an image output unit to output second read data of a second side of a document sheet at each time when a storage unit stores the second read data output from a second image reading unit on a line-by-line basis, controls the storage device to store first read data of a first side of the document sheet during a time period from a time when a leading end of the document sheet reaches a reading position of a first image reading unit to a time when a trailing end of the document sheet passes through a reading position of the second image reading unit, and controls the image output unit to sequentially output the first read data stored on the storage unit, after completely outputting the second read data.