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.