The image reader according to the present invention is able to set the accurate threshold level free of influence of dirt and dust even when the reference “white” level is requested for shading correction during the acceleration while the carriage is being accelerated without increasing the averaging frequency when the reflected light from the white board is read for shading correction. In addition, because the distance between the head end of the transparent glass and the home position as well as the distance between the home position and the reading window are shortened, the size of the image reader is reduced.