PURPOSE: To make it possible to carry out parts layout in a wiring board containing a wiring length limiting net efficiently by displaying a wiring length limiting area that is an aim for protecting a virtual wiring length from exceeding a limit value regarding a wiring length limiting net for parts.

CONSTITUTION: When regarding pins 22-25 belonging to a wiring length limiting net that layout object parts 15 have an allowable value is positive that was stored in a processing box for the wiring length limiting net to which the relevant pins 22-25 belong, the processing box displays wiring length limiting areas 30 and 31. It is assumed that wiring length limiting areas 30 and 31 are squares having a diagonal line two times the allowable value, and center positions thereof are those where object pins 18 and 20 corresponding to relevant pins are located. If relevant pins 22-25 of the layout object parts 15 are included in wiring length limiting areas 30 and 31, the virtual wiring length, after wiring is determined, cannot exceeds a limiting value. Further, if in the processing box the virtual wiring length exceeds the limiting value, rat nests 26-29 are displayed in error indicating color.