Abstract: In an image scanner, a lens position control mechanism and method converts a reading line density by changing over lenses. A base rotatable in a plane parallel to an original table is provided, and the lenses are supported on the base so that their optical axes are in a plane parallel to the base surface and intersect each other at a pivot. A proper kind of lens can be arranged at a proper position by controlling the rotation of the base in such a manner that a reference pattern is provided at a position at which it is readable by an image receiving element through the lens and a lens existing in an optical path is judged on the basis of a received image of the reference pattern to confirm the position of the lens. Thereby, an image scanner capable of changing over lenses at a high precision can be obtained without increasing the height of the image scanner.