The present invention relates to a method for controlling an image scanner for reading images by moving an image sensor. Specifically, the present invention relates to a control method for detecting the boundary between a white region and a black region formed at a predetermined place in accordance with a signal output from the image sensor and determining the home position of the image sensor in accordance with the position of the boundary. Particularly, the present invention is an invention for providing a control method for preventing erroneous recognition due to influence of external light.The above objects are achieved by methods of the present invention such as a method for confirming whether, when an output signal probably showing a black region is obtained, the same detection result is obtained again at another position in the black region, a method for excluding the vicinity of a housing end subject to external light from a read range, and a method for previously generating a threshold for detecting a black region in accordance with a signal output from an image sensor to use the value of the signal.