For functional blocks of an IC, each functional block comprising a plurality of macroblocks, each macroblock comprising a plurality of basic cells, a mask pattern is patterned in accordance with a layout design by using its connection data of the macroblocks in each functional block and its placement data of the basic cells in each functional block. Use of the placement data in addition to the connection data makes it possible to regularly and systematically arrange the basic cells in each functional block to achieve shortest possible connections in each fundamental block and a narrowest possible area of each functional block. In order to put a CPU in operation of patterning the mask pattern, an operating system comprises for read by the CPU first and second memories loaded with the connection and the placement data, respectively.