Patent ID: 8134753

Claim:
An original size detecting apparatus comprising: an original platen; a light source that illuminates an original placed on said original platen; a reading device that reads the original based on light reflected from the illuminated original; a first detecting device that detects whether or not a dimension of the original in a sub-scanning direction is equal to or smaller than a predetermined dimension; a second detecting device that detects a dimension of the original in a main-scanning direction orthogonal to the sub-scanning direction; and a determining device that determines a size of the original based on results of detections by said first and second detecting devices, wherein said second detecting device carries out a first original size-determining process when at least one of output values of said reading device at a plurality of positions in the main-scanning direction with said light source turned off is smaller than a predetermined output value, and carries out a second original size-determining process when all the output values of said reading device at the plurality of positions in the main-scanning direction with the light source turned off are larger than the predetermined output value.