Workpiece data involving a product shape and having a smallest diameter for lathe turning around a turning axis is selected from the workpiece database, by comparing dimension data of a workpiece model with dimension data of a product model in a state in which the product model is arranged on the turning axis and the workpiece model is arranged so that a center axis of each workpiece matches a center of the turning axis. The workpiece model for lathe turning is created based on the selected workpiece data.