A hardware accelerator for a raster-based design-rule checking called BITDRC for a bit-mapping CAD system is described. BITDRC is a special-purpose hardware accelerator which performs design-rule checking for the Manhattan layout style VLSI circuis, much faster than the software checking which belonged to the bit-mapping CAD system before. The bit-mapping CAD system had effectively been developed for both of educational and VLSI design purposes, and just needs only a personal computer as a compact working environment. The proposed hardware architecture is rather simply and characterized by the bit-mapping CAD system where it works on. The hardware architecture and checking algorithm have been confirmed by implementing a bread-board prototype using discrete components. As a result, the processing time of BITDRC is speeded up as much as 500 times faster than the original software and takes only 4 seconds for checking every rule on a(15001500) grids layout pattern. BITDRC performs the error checking together with the data scanning that makes it can be as an on-line design-rule checker for the bit-mapping CAD system. Finally, the physical layout of BITDRC has been designed using a conventional CMOS technology.