To quickly and accurately determine the radius of curvature of a route through simple arithmetic processes by directly using point data from a navigation system.

A three-point detecting part 11 detects three points (P1, P2, P3) from road point data which are input from a navigation system 2, a P1-to-P2 distance computing part 12 computes the distance of a straight line connecting P1 to P2, a P2-to-P3 distance computing part 13 computes the distance of a straight line connecting P2 to P3, and a distance judging part 14 determines whether the distance of each straight line is long or short. A mid-point computing part 15 computes half the distance of a short straight line to determine the position of a mid-point on the straight line, while a mid-point equidistant point computing part 16 determines a mid-point equidistant point at the position on a long straight line which is away from P2 by half the distance of the short straight line. A radius-of-curvature computing part 17 determines, as the center position of a curve, the position of an intersection between a straight line crossing the short straight line at right angles at the mid-point and a straight line crossing the long straight line at right angles at the mid-point equidistant point to compute the radius of curvature. The radius of curvature is corrected by a correcting part 18 under necessary conditions and is output.