PROBLEM TO BE SOLVED: To provide a bone prosthesis capable of retaining an adaptive state to a bone of a patient for a long period of time and inhibiting a feeling of discomfort in the external appearance, and a design method thereof.SOLUTION: A bone prosthesis 1 is placed to be received in an inclined edge part 103 that includes an outward-facing surface 105 facing the outside of a cranial bone 101 in a defective part 102 formed on the cranial bone 101 of a patient 100. The design method of the bone prosthesis 1 includes the steps of: setting first and second profile lines L1 and L2 having a shape corresponding to the shape of the edge part 103 and setting a facing surface 4. The first profile line L1 and the second profile line 2 are located at different positions in the thickness direction A1 of the bone prosthesis 1. The facing surface 4 includes the first and second profile lines L1 and L2 and faces the edge part 103.
