1. An image processing apparatus comprising: an image acquisition unit configured to acquire an image; a feature amount acquisition unit configured to acquire an image feature amount including an alternating current component and a direct current component from each of a plurality of partial regions in the acquired image; a holding unit configured to hold, as a background model, an image feature amount of an alternating current component and a direct current component of a background image for each of the partial regions; a comparison unit configured to compare, for each of the partial regions, the image feature amount of the background model with the image feature amount of the acquired image; first and second update units configured to update the alternating current component and the direct current component for the partial region of the background model, respectively, if the partial region of the acquired image is similar to a corresponding region of the background model based on the comparison result of said comparison unit, using corresponding components for the partial region of the acquired image; and a detection unit configured to detect, using the background model updated by said first and second update units, for each of the partial regions, a region where a target object to be detected exists, wherein a degree of update of the direct current component updated by the second update unit is larger than that of the alternating current component updated by the first update unit.