An approximate curve generation method according to the present invention generates an approximate curve as smooth as an original curve from point group data obtained by approximating a curve shape, and includes: an approximate polygon shape range determining step of sectioning point group data for each line segment of an approximate polygon shape which is generated when the point group data approximating the curve shape are obtained, and sorting the point group data close to a section boundary as valid point group data, and the point group data inside the valid point group data as invalid point group data; and an approximate curve generating step of regarding only the valid point group data, and generating an approximate curve passing through or close to the regarded valid point group data as an approximate curve to be finally obtained.