1. A photographic subject tracking method comprising: calculating similarity levels of images input sequentially to a template image; determining a photographic subject position in each of the input images based upon the similarity levels; tracking the photographic subject position through the input images; judging whether reliability of the similarity level calculated for the photographic subject position is high or low; and changing ways of at least one of calculating the similarity levels, determining the photographic subject position and tracking the photographic subject position, if the reliability of the similarity level is judged to be low, wherein: the similarity levels are calculated by setting a search target area in each of the input images, shifting a search frame to a plurality of positions within the search target area and calculating similarity levels of images within the search frame at each of the positions to the template image; the photographic subject position is determined to be at a position of the search frame at which a highest similarity level is calculated; if an absolute value of a difference between an average of the similarity levels and the highest similarity level is greater than a predetermined threshold value, the reliability is judged to be high, and if the absolute value of the difference is equal to or less than the threshold value, the reliability is judged to be low; and if the reliability is judged to be low: an areal size of the search target area over which the search frame is to be shifted is adjusted in correspondence to the similarity level calculated for the photographic subject position; the similarity level is recalculated through template matching of the image within the search frame and the template image executed by shifting the search frame within the search target area with the adjusted areal size set within a current input image; and a position of the search frame with a highest recalculated similarity level within the input image is designated as the photographic subject position.