An image reading apparatus capable of changing a main reading unit at an appropriate time point such that a plurality of reading units can be equally used, and a method of controlling the same. The image reading apparatus includes an automatic document feeder, a first reading unit to read a first side of a document fed by the automatic document feeder and to function as a main reading unit, a second reading unit to read a second side of the document fed by the automatic document feeder; and a control unit to change the main reading unit from the first reading unit to the second reading unit if the number of times La of usage of the first reading unit reaches a reference value. The control unit compares the number of times La of usage of the first reading unit with a predicted lifetime Lt of the first reading unit and changes the main reading unit to the second reading unit if it is determined that the number of times La of usage of the first reading unit reaches a predetermined percentage of the predicted lifetime of the first reading unit. The control unit determines whether the main reading unit is changed or not, based on frequencies of use of the first reading unit and the second reading unit.