1. A detector for detecting a specified image in an input image, comprising: an area determination unit operable to determine, in the input image, a detection target area in which the specified image potentially exists; a setting unit operable to set positions of a plurality of matching target ranges substantially in the detection target area, each of the matching target ranges being a predetermined size, so that the matching target ranges cover the detection target area, and each matching target range overlaps a neighboring matching target range by a predetermined overlap width; and a matching unit operable to detect the specified image by matching a portion of the input image encompassed by each matching target range set by the setting unit and a template image for detecting the specified image.