1. An individual identification system which identifies individuals by comparing irises thereof, comprising: a first closed curved-line preparation circuit preparing a first closed curved-line similar to the iris; a first similarity calculation circuit calculating a first similarity between the iris and the first closed curved-line; a first judgment circuit judging whether or not the first similarity satisfies a first predetermined threshold similarity value; a first decision circuit deciding that the first closed curved-line is representative of the outline of the iris upon judgment that the first similarity satisfies the first predetermined threshold similarity value; a second closed curved-line preparation circuit preparing a second closed curved-line similar to the iris upon judgment that the first similarity does not satisfy the first predetermined threshold similarity value; a second similarity calculation circuit calculating a second similarity between the iris and the second closed curved-line; a second judgment circuit judging whether or not the second similarity satisfies a second predetermined threshold similarity value; and a second decision circuit deciding that the second closed curved-line is representative of the outline of the iris upon judgment that the second similarity satisfies the second predetermined threshold similarity value, said second decision circuit selecting a second closed curved-line that is the most similar to the iris among a plurality of second closed curvedlines upon judgment that no second similarity satisfies the second predetermined threshold similarity value, and deciding that the selected second closed curved-line is representative of the outline of the iris.