1. A subject tracking apparatus which tracks a specific subject in an image over a plurality of images input thereto in time series, comprising: computation unit configured to obtain, with respect to each of a plurality of reference images registered in registration unit, a maximum degree of correlation of the input image and a region which is included in the input image and has the maximum degree of correlation; and first determination unit configured to determine a region of the specific subject in the input image from the regions obtained by said computation unit, wherein the plurality of reference images include a first reference image which is based on a region specified as the region of the specific subject in a first input image among the plurality of input images and is not updated, and a second reference image which is updated based on a region determined as the region of the specific subject by said first determination unit in each of a plurality of second input images, that are input to the subject tracking apparatus in time series later than the first input image, among the plurality of input images, and said first determination unit determines that a region with a maximum degree of correlation with the first reference image is the region of the specific subject in the input image if the maximum degree of correlation obtained with respect to the first reference image by said computation unit is not less than a predetermined threshold, and determines that a region with a maximum degree of correlation with the second reference image is the region of the specific subject in the input image if the maximum degree of correlation obtained with respect to the first reference image by said computation unit is less than the predetermined threshold.