Image reader and image reading method

The present invention provides an image reader which comprises: a two-dimensional image sensor; a micro displacement mechanism which minutely shifts the image sensor from an initial position on a plane normal to an optical axis; and a detector which detects images before and after shifting. The micro displacement mechanism displaces the image sensor until the relative distance coincides with a specified value within an allowable range. The specified value is digitized, and its relative distance can completely coincide with the specified value. The image sensor shifts relative to an original with a displacement value smaller than the preset shift amount initially, and the actual relative distance is detected at the time. From the next time, the relative distance can be approximated to zero at once.