1. An image processing method for detecting a linear object on a gray-scale image, wherein edge pixels of the gray-scale image are extracted, and while considering the extracted edge pixels sequentially, an edge pixel verified to have a density gradient direction opposite to the edge pixel under consideration is searched for within a predetermined range in the direction orthogonal to the edge direction from the edge pixel under consideration, and when an edge pixel satisfying the verification conditions is extracted, a line segment is set in parallel to the edge direction of the edge pixel under consideration and passing through a predetermined position from the extracted edge pixel to the edge pixel under consideration, and wherein an image indicating the result of setting the line segment is displayed as a linear object detection result when there is no edge pixels to be considered.