1. An image processing method using an image processing device which detects a specific detecting object from an image including a plurality of two-dimensionally arranged pixels, comprising: setting a plurality of candidate elliptical areas, each defined by using a center and lengths in both a first direction and a second direction orthogonal to each other, that become areas of a detecting object in the image; comparing luminance of the pixels inside the candidate areas with the luminance of the pixels outside the candidate areas, for each of the set candidate areas; detecting an area suitable to the detecting object out of a plurality of the set candidate areas on the basis of the comparison result; setting, as a circumscribed rectangle, a rectangle that circumscribes the area suitable to the detecting object and has one side parallel to the first direction; setting a reference luminance based on distribution of luminance of pixels which are included in the circumscribed rectangle; calculating a central moment around a center of the distribution of the pixels with higher luminance than the reference luminance in the circumscribed rectangle; and detecting an inclination of the detecting object in the first direction in the image.