1. An object detection apparatus, comprising: an image input unit including circuitry configured to receive an image; an image feature quantity extraction unit configured to extract a predetermined image feature quantity from the image to generate a feature-quantity extraction image; and a symmetry evaluation unit configured to set, for every processing target pixel in the feature-quantity extraction image, a symmetry evaluation area for evaluating symmetry in a first direction in the image in such a manner that the symmetry evaluation area is symmetrical with respect to a central axis in a second direction orthogonal to the first direction, and calculates a weighted value resulting from weighting a value indicating a correlation value by an image feature quantity for a group of pixels included in the symmetry evaluation area and located symmetrical with respect to the central axis, the correlation value indicating a correlation between image feature quantities of the group of pixels, while varying a size of the symmetry evaluation area to obtain a symmetry evaluation value indicating a degree of the symmetry in the first direction, and configured to calculate the symmetry evaluation value by varying a value of w either within a first range or a second range that is different from the first range, based on whether a value of a row indicating a position of the target pixel in the first direction is greater than or less than H/2, where w is a half of a width of the symmetry evaluation area in the first direction and H is a number of pixels in the first direction of the image.