Designing of a coping for proper working is made possible. Based on three-dimensional data on a model of an abutment tooth to which a dental prosthesis is applied stored in RAM (Random Access Memory) and on three-dimensional position information about a margin line of the model stored in another RAM, on a face containing a reference axis of the model, three-dimensional position information about a side face of a coping is determined so that a specified angle is formed between a margin line and a surface of the model and three-dimensional position information about the side face of the coping on each of a plurality of different faces containing the reference axis over an entire circumference in a direction surrounding the reference axis is detected by a coping processing section.