An image reading apparatus capable of increasing the detection accuracy of the size of an original and eliminating the need for an image sensor dedicated to detection of the size of an original. A first reading unit has a first light source disposed on one side of a conveyance path of an original for emitting light to one surface of the original. A second reading unit has a second light source disposed on the other side of the conveyance path for emitting light to be received by the first reading unit, an optical opening for causing the light emitted from the second light source and subsequently received by the first reading unit to pass therethrough toward the first reading unit. The original is detected on the basis of the resultant output of the first reading unit upon receipt of the light emitted from the second light source.