An image reading apparatus that can, even when foreign matter exists on an original platen glass, determine a position less affected by the foreign matter, as an image reading position. In the image reading apparatus, an original is conveyed to a reading area on the original platen glass, and a reading unit reads the original at a reading position within the reading area. Image data is read at a plurality of candidate reading positions within the reading area in a state where the original is not conveyed, and foreign matter adhering to the candidate reading positions is detected. The number of pieces and the size of each piece of the detected foreign matter are measured, and a reading position for reading the original is determined from the plurality of candidate reading positions using a result of the measurement.