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