A plurality of anchor points at which a curve to be estimated passes sequentially through are set. Four anchor points which are successional are selected from the plurality of anchor points. First and second control points are set based on the selected anchor points. Quadratic Bezier curves connecting between the selected anchor points are calculated based on the selected anchor points and the first or second control point. Coordinate values of the curve to be estimated are estimated based on the quadratic Bezier curves.