A CAD apparatus designing patterns of a printed board. The apparatus includes a signal wiring pattern detecting unit for detecting a signal wiring pattern, and a guard ground detecting unit for tracing the signal wiring pattern along the longitudinal direction thereof, in order to detect whether or not a guard ground exists within a distance from the signal wiring pattern to a predetermined first discriminant value, identify a non-guard ground section within the distance to the first discriminant value, and to detect whether or not a guard ground exists within another distance from the signal wiring pattern to a second discriminant value, which is larger than the first discriminant value. The apparatus also includes a return path judging unit for judging an error of guard ground when no guard ground exists within the distance to the second discriminant value, thereby quickly checking a return path without mistake.