Document size detecting method and apparatus

An original document reading apparatus (1) includes a LED array (2b) having an alignment of a plurality of point light source elements (21) in a main scanning direction beneath an original document table (2a), a moving device (2i, 2j) that moves the LED array (2b) in a sub scanning direction, and a photo acceptance unit (2g) that accepts a light reflected from the original document (2k) and determines a size of the original document (2k) based upon a reaction' of a pixel to the light. The plurality of point light source elements are divided into a plurality of blocks (a to g) to be turned on and off in turn per a block starting from a block (d) corresponding to an original document minimum size to a block (g) corresponding to an original document maximum size.