A mechanism for and a method of controlling the lens position in an image scanner in which the density of read lines can be varied by selecting one of lens. A base is rotatably provided in a plane parallel to a table on which an original is placed. The lens is supported on the base in such a way that the optical axis of each lens is parallel to the base surface and intersects the rotation axis. A reference pattern is provided at a place where an image receiving device can read it through the lens. On the basis of the received image of the reference pattern, the lens in the optical path is identified, and its position is determined. The rotation of the base is thus controlled to set an appropriate lens in an appropriate position. In such a way, an image scanner such that the lens can be changed in a high precision is provided without increasing the height.