1. An optical navigation sensor device comprising: an image pixel array for calculating analog voltage values of pixels thereof respectively corresponding to relative intensities of reflected light beams incident to the pixels from a support surface, on which the sensor device is laid; an analog/digital (A/D) converter for converting the analog voltage values of the pixels received thereto into corresponding digital voltage values, respectively, thereby producing a current image of the support surface; a pre-processor for sequentially receiving the digital voltage values from the A/D converter for all pixels of the current support surface image in accordance with a predetermined timing signal, performing a pre-process for the current support surface image, thereby producing a pre-processed current image consisting of pixels each having a 2-bit digital voltage value, and extracting, from the pre-processed current image, a pre-processed current central image having a predetermined pixel array; and a motion coordinate calculator for setting, as X/Y-axis reference image candidates, the pre-processed current central image received from the pre-processor, calculating an X-axis motion vector while overlapping a previously-set X-axis reference image with the pre-processed current image, thereby calculating an X-axis motion coordinate value of the pre-processed current image, and calculating a Y-axis motion vector while overlapping a previously-set Y-axis reference image with the pre-processed current image, thereby calculating a Y-axis motion coordinate value of the pre-processed current image.