1. An input device identification method applicable to an input device comprising at least three contact portions to be in contact with a touch panel, the input device identification method comprising the steps of: a) storing a distance between two contact portions in the input device as a contact portion distance for each of a plurality of combinations made by selecting two contact portions from respective contact portions of the input device; b) acquiring coordinates of the respective contact portions of the input device placed on the touch panel; c) calculating a distance between acquired two coordinates as a coordinate distance for each of a plurality of combinations made by selecting two coordinates from respective acquired coordinates; d) calculating a similarity ratio between a shape formed by the respective acquired coordinates and a shape formed by the respective contact portions on the basis of the calculated coordinate distances and the stored contact portion distances; and e) identifying the input device in contact with the touch panel by comparing the stored contact portion distances and the calculated coordinate distances after converting either one of the stored contact portion distances and the calculated coordinate distances to the same scale as the other on the basis of the similarity ratio.