A level difference or a uncut portion is prevented from being left between adjoining worked regions without forming any useless worked region. A lathe sectional shape forming unit (222) produces a sheet model of a lathe sectional shape in an X-Z plane on the basis of the solid model of a lathe shape formed by a lathe shape forming unit (220). A first/second step working sectional shape forming unit (227) produces sheet models of the worked sectional shapes of the first step and the second step on the basis of the sheet model of the lathe sectional shape, a step dividing position and an overlap amount. A first step disused shape deleting unit (229) and a second step disused shape deleting unit (230) delete the shape needing no work, from the sheet models of the worked sectional shapes of the first step and the second step.