According to one embodiment, a preoperative planning method performed by a computer for hip replacement arthroplasty, includes a base jig setting step of setting a three-dimensional image of a base jig having three or more columnar supports which abut on three or more reference points of the pelvis with respect to the three-dimensional image of the pelvis obtained at the image reconstruction step, and a parameter acquisition step of parameterizing and acquiring a direction of an indicator which runs through a predetermined position on the base jig in the three-dimensional image of the base jig set at the base jig setting step and becomes parallel to the installation direction of the joint prostheses determined at the joint prostheses determination step.