There are provided an image processing method, and an image processing system, an image processing device, and a computer program product capable of detecting a position of a fringe portion of a detecting object such as an outline of a face of a driver with high precision, even when a variation of illuminance occurs locally in a system using, for example, an on-vehicle camera mounted on a vehicle for imaging the face of the driver. Luminance of pixels arranged in a first direction such as a horizontal direction of an image is subjected to quadric differential, and data based on a quadratic differentiation result is accumulated in a second direction such as a vertical direction, and based on an accumulated result, a position in the first direction of the detecting object in the image is detected.