1. An image processing apparatus for detecting a contour circle as contours of a subject based on image data provided by an image acquiring unit, said image processing apparatus comprising: a processing target area specification section for specifying a processing target area; a circumferential direction specification section for specifying the circumferential direction of the contour circle in the processing target area; an edge detection region specification section for specifying three or more regions made different in position in the circumferential direction in the processing target area as edge detection regions based on the processing target area and the circumferential direction; an edge position identification section for identifying edge positions with respect to the radial direction of the-contour circle based on the intensity distribution in the edge detection regions; and a contour circle determination section for determining the contour circle based on the edge positions identified relative to the edge detection regions.