An image reader includes a lamp that irradiates light onto a document, a first carriage that moves the lamp along the document to optically scan the document, a CCD that receives light reflected from the document and prepares an image data indicative of an image of the document. A size sensor detects a length of the document in an auxiliary scanning direction, and an IPU that detects a length of the document in the main scanning direction from the image data. A CPU monitors whether a size of the document has changed, based on the detected by the size sensor, and controls irradiation of the light by the lamp based on the result of monitoring.