A circuit board design aid is achieved by generating a shield pattern for a wiring pattern including a pattern element in a circuit board by increasing a width of a geometry of the pattern element by an amount corresponding to a shield pattern spacing set as a preset pattern generation condition. A prohibition region is generated based on a geometry of an element for which a clearance check is to be performed located around the wiring pattern and a clearance condition between the element for performing a clearance check and the wiring pattern. Then, the shield pattern is generated by excluding the geometry of the prohibition region from the geometry of the basic shield pattern element.