1. A fabrication aiding apparatus comprising: a shape data storing means to store three-dimensional shape data on a model of an abutment tooth to which a dental prosthesis is applied, an angle input accepting means to accept an input of specification of an angle formed between a line defining a contact point and a reference axis on a face containing said reference axis in said model, a contact point detecting means to detect, based on the three-dimensional shape data, three-dimensional position information about a contact point with the line that forms a specified angle with the reference axis on a face containing the reference axis in said model, and a margin line determining means to determine, based on coordinates of the contact point, three-dimensional position information of a margin line in said model of said abutment tooth.