A manufacturing method of a bone cutting assist device includes generating a bone cutting assist device model representing the bone cutting assist device including a fitting surface that fits a surface of a bone model, a cutting slit that guides a cutting jig, a first guide hole for inserting a first rod model, and second guide holes for inserting second rod models. The manufacturing method also includes outputting three-dimensional manufacturing data representing the generated bone cutting assist device model, and manufacturing the bone cutting assist device according to the outputted three-dimensional manufacturing data.