Graphic data of design objects designated by an input device are retrieved from a design object data memory and displayed on a display device. A restrictive item applied to the design object displayed on the display device and redesignated by the input device is read from the design object data memory. Specification data of the designated design objects are also read from the design object data memory. A condition part of the design reference is made based on the redesignated design object and the selected specification data, and an end portion of the design reference is made based on the redesignated design object, the selected restrictive item and a reference value entered by the input device. The design reference including the condition part and the end portion is displayed on the display device and stored into a design reference memory.