An object detection apparatus for obtaining information about a size of an object included in an image, comprising: an image input unit configured to receive the 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, characterized in that the symmetry evaluation unit is configured to, for every processing target pixel, calculate 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, for each symmetry evaluation area of varied size, a symmetry evaluation value indicating a degree of the symmetry in the first direction, and determine a width of the symmetry in the first direction as the information about the size of the object based on the size of the symmetry evaluation area among the symmetry evaluation areas of varied size, which corresponds to the symmetry evaluation value indicating the highest symmetry in the first direction. an image input unit configured to receive the 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, characterized in that the symmetry evaluation unit is configured to, for every processing target pixel, calculate 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, for each symmetry evaluation area of varied size, a symmetry evaluation value indicating a degree of the symmetry in the first direction, and determine a width of the symmetry in the first direction as the information about the size of the object based on the size of the symmetry evaluation area among the symmetry evaluation areas of varied size, which corresponds to the symmetry evaluation value indicating the highest symmetry in the first direction. for every processing target pixel, calculate 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, for each symmetry evaluation area of varied size, a symmetry evaluation value indicating a degree of the symmetry in the first direction, and determine a width of the symmetry in the first direction as the information about the size of the object based on the size of the symmetry evaluation area among the symmetry evaluation areas of varied size, which corresponds to the symmetry evaluation value indicating the highest symmetry in the first direction.