The present invention provides an image reading apparatus used for automatically determining whether first and second reading units, which are opposed to each other with a document interposed therebetween, are normally operated, and a method of controlling the same. The image reading apparatus includes a duplex path to guide the document, which is read by the first reading unit and the second reading unit, to be transported to the first reading unit and the second reading unit again. The document is subjected to a primary reading operation using the first reading unit and the second reading unit, and the document, which was subjected to the primary reading operation, is then guided via the duplex path and passes through the first reading unit and the second reading unit. At this time, the document is subjected to a secondary reading operation using the first reading unit and the second reading unit, and a control unit compares image information obtained by the first reading unit and the second reading unit in the primary reading operation with image information obtained by the first reading unit and the second reading unit in the secondary reading operation. The control unit also determines whether the first reading unit and the second reading unit are normally operated.