A shape measurement data is acquired by measuring along a spiral measurement path a lens shape which is machined along a spiral machining path. An interpolated shape measurement data at intersecting points of a radial line passing through a center of the lens shape and the spiral measurement path is acquired by interpolating the shape measurement data, and a compensated machining amount for removing a machining error at each of the intersecting points (the machining points) is calculated from the interpolated shape measurement data by interpolation. Further, a machining point compensated machining amount at each of the machining points on the spiral machining path is calculated from the calculated compensated machining amount, and a compensated machining path is created on the basis of the machining point compensated machining amount.