An image reading apparatus capable of accurately detecting abnormal pixels caused by a contaminant such as dirt, stain, or flaw on a moving document reading glass being read. An image processing unit of the image reading apparatus performs a first abnormal pixel detection process to detect pixel signals representing abnormal pixels and output from image sensors for respective colors when a reading unit reads a guide plate in a main scanning direction in a state before an original passes through a moving document reading position, performs a second abnormal pixel detection process to detect pixel signals representing abnormal pixels and output from the image sensors when an original being conveyed is read by the reading unit in the main scanning direction, and detects a main scanning direction of abnormal pixels based on results of these detection processes.