ORIGINAL DOCUMENT SIZE DETECTION DEVICE

An original document size detection device characterized by comprising a document reading table, a light source which irradiates an original document placed on the document reading table, a cover body which covers the original document, a light detection unit which detects a reflected light that a light from the light source is reflected by the original document, an angle detection unit which detects an opening angle of the cover body and has a first photo sensor, a second photo sensor, a third photo sensor, and a shield plate, and a determination unit which determines the size of the original document in a sub-scanning direction when the opening angle of the cover body is equal to an angle al and the size of the original document in a main scanning direction when the opening angle of the cover body is equal to an angle α2 (α1>α2) based on an output signal of the light detection unit and an output signal of the angle detection unit.