To form tool paths with a uniform margin corresponding to both the shape of a product and the shape of material, enhance machining efficiency and prolong tool life.

The product shape PS having a recess in a portion thereof and the material shape MS are extracted. The material shape MS is offset outwardly by the tool radius and the product shape is offset onto or outward of the offset line M of the material shape. Offsets are made from the offset line P of the product shape obtained while allowing for a maximum margin and residual amounts inwards to form the plurality of tool paths T1, T2 and T3. The material shape AM after cutting is made from the tool paths formed is determined and is offset outwardly of the material shape by the tool radius. Thereafter, these processes are repeated as the tool diameter is varied if necessary, until an uncut shape is eliminated or the margin becomes no longer available.