When a signature is registered, a signature in Japanese characters, for example, `` (`Fujitsu`), or a signature in English characters, for example, `Fujitsu` is input n times through an input device of an input equipment of a signature authentication system, and each element of the shape of handwriting, writing speed, writing acceleration, and writing pressure is computed. A combination of nC2 is set for each element of the data input n times, the reciprocal of each similarity is obtained, an average value of the similarity is computed for each element, a weight is determined for each similarity from the average value, a weighted average value of the four elements is computed and defined as the total similarity of the registered signature. In an authenticating process, signature data to be compared is input/output m times. In the same method, the total similarity of a signature to be compared is obtained.