Abstract: A method and an apparatus for compensating illuminance error of a light source is adapted to be used in a transmission type scanner and utilizes the light signal directly transmitted from the light source to an image pickup device of the scanner without passing through any scanned object as an illuminance calibration reference to compensating illuminance error of the light source. The calibration light signal can be transmitted from the light source to the image pickup device through a light transmission region arranged beside a scanning region.