A method of manufacturing a patient-customized tibial element for use in artificial knee joint surgery. A tibial element data table regarding sizes of proximal tibial prostheses is formed. Standard molds able to cover the tibial element data table are manufactured. A standard mold is selected from the standard molds in order to manufacture a patient-customized proximal tibial prosthesis. A proximal tibial prosthesis fitting a patient is formed using the selected standard mold.