1. An image processing apparatus detecting an edge of a detection-target subject imaged by an image pickup device having line-defect pixels, the apparatus comprising: rotating means rotating the image pickup device or the detection-target subject; imaging control means imaging the detection-target subject with the image pickup device, with the image pickup device or the detection-target subject being rotated by the rotating means at a predetermined angle so that the edge of the detection-target subject is not parallel to each of horizontal and vertical scanning lines of the image pickup device; and image processing means detecting the edge of the detection-target subject by performing sub-pixel processing on a gray-scale image obtained by performing correction by interpolation on a luminance of each of the line-defect pixels with luminances of pixels adjacent to both sides of the line-defect pixel.