An original size detecting apparatus is capable of carrying out accurate original size detection without erroneous detection of the size of an original due to the influence of external scattered light. A CCD reads reflected light of light irradiated from a light source onto an original placed on an original platen glass. Optical sensors detect two open states of the original presser plate, and a scanner controller performs predetermined control corresponding to each of the open states of the original presser plate detected by the optical sensors, to determine the size of the original based on the result of the reading by the CCD.
