First of all, size data (MD), (BD), (BH) and (MW) of a tire are input. Next, a ratio (b/a) of a major axis radius (b) to a minor axis radius (a) of an ellipse (18) is input. Then, constants (S), (a) and (b) of an involute function are calculated. Based on the involute function having these constants, a profile function (22) from a central position in a lateral direction to a maximum width position is determined over an X-Y plane. Subsequently, the profile curve (22) is divided into a large number of segments. Next, each of the segments is caused to approximate to a circular arc. In the approximation, a radius of curvature is determined in such a manner that the adjacent circular arcs are tangent to each other. Thus, a profile is designed.