1. A method for sensing by a linear image sensor a two-dimensional image of an object to be sensed, the method comprising the steps of: detecting a position of the object by a position sensor having a first resolution; picking up an image of the object being projected in a direction (V-scan direction) generally perpendicular to an internal scan (H-scan) direction of the linear image sensor in synchronism with relative movement between the object and the linear image sensor; periodically switching a pixel size along the first direction between a plurality of predefined pixel sizes during the relative movement between the object and the linear image sensor in accordance with information of the relative movement detected by the position sensor; and producing from an output of the linear image sensor a two-dimensional image of the projected image having a second resolution of the object to be sensed; wherein the second resolution is a higher resolution than the first resolution.