Abstract: An image scanner is comprised of a light source for illuminating an original document, an image sensor for receiving a reflected light from the original document and converting the same into an output signal of image data, a device for relatively moving the original document and the image sensor, an iris mechanism provided on an optical path between the original document and the image sensor, a reference reflection plane provided such that a light from the light source is reflected and the reflection light becomes incident on the image sensor, a comparing circuit for comparing a level of an output signal from the image sensor with a reference level when the image sensor receives the light from the reference reflection plane, and a device for correcting a dynamic range of the image scanner by controlling the iris mechanism on the basis of the compared output.