An original document reading apparatus includes a plurality of point light source elements (e.g. a LED array) in a main scanning direction beneath an original document table, a moving device that moves the point light source elements in a sub scanning direction, and a photo acceptance unit that accepts light reflected from the original document and determines a size of the original document based upon a reaction of a pixel to the light. The plurality of point light source elements are divided into a plurality of blocks to be turned on and off in successive turns per a block starting from a block corresponding to an original document minimum size to a block corresponding to an original document maximum size.