Image reader and image reading method

The present invention provides an image reader comprising: a two-dimensional image sensor; a micro-displacement mechanism which minutely shifts the image sensor from an initial position on a plane to perpendicularly intersect with an optical axis; and a detector which detects the relative distance between images before and after pixel 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. By initially displacing the image sensor relative to an original with a value smaller than a set shift amount, and detecting the actual relative distance at the time, the relative distance can be approximated to zero at once from the next time.