A pattern-element extractor (51), one of functions implemented by a computer, extracts a plurality of pattern elements from a reference image, a region of which is displayed on a display (45). An input part (46) accepts an operator's selection of one out of the plurality of pattern elements as a reference pattern element, and a distinguishability checker (52) checks whether the selected pattern element is distinguishable from the other pattern elements and usable as the reference pattern element for pattern matching. A result of the check is notified to the operator, and an appropriate one out of the pattern elements in the reference image can be selected as the reference pattern element, by referring to the result of the check made by the distinguishability checker (52). Thus, misrecognition of the reference pattern element used for detecting a position of a target image relative to the reference image in pattern matching is prevented.