To reduce manual correcting operation by generating three-dimensional model data for projection conversion from geometric information of three- dimensional model data and prepared alteration rule information and generating two-dimensional drawing data.

A central processing unit(CPU) 30 inputs a three-dimensional model data file 10, and generates and outputs drawing data conforming with drawing standards as a two-dimensional drawing data file 20. In this case, a model for shadow conversion is generated temporarily in a model generating process for projection conversion according to alteration rule information 41 and held in a work area 43 of a storage device 40, and in a projection converting process, the two-dimensional drawing data 20 are generated from the model for projection conversion. Thus, the easy-to-see two-dimensional drawing data 20 conforming with the drawing standards can automatically be generated and the three-dimensional model data for the model for projection conversion become simplified model data, so the amount of data as objects of projection conversion can be decreased to enable high-speed processing.