In an image reading apparatus, the invention provides a method for detecting foreign matter which may adhere to a white reference plate, and correcting a reference value when foreign matter is detected. The method includes computing an average of read values, comparing an absolute value of difference between the computed average value and data of the next read position to a predetermined threshold, and when a determining unit determines that foreign matter adheres to the reference plate, replacing the average value used for the determination of whether or not the foreign matter adheres to the reference plate with the read data.