A method includes reading an image of a document obtained by a reading unit exposing the document to light with an exposure unit, causing the reading unit to perform first reading processing for reading first information obtained by causing the exposure unit to perform exposure with a first light amount, and second reading processing for reading second information obtained by causing the exposure unit to perform exposure with a second light amount that is smaller than the first light amount while no document is placed at a reading position of the reading unit, and detecting a foreign matter existing at the reading position according to a difference between the first and second information.