respective reduction ratios becoming new lower level cells after that a cell which is one level higher than the cells 31 through 34 is compaction processed in step 25 in other words the upper level cell 35 is compaction processed so that it may be adapted to the new design standards at this time the compaction processing of the upper level cell 35 is conducted in accordance with the inter cell connection information previously extracted in step 22 so that the inter cell connection of the lower level cells before the compaction processing may be retained in this way new lsi mask pattern data comprising compaction processed cells are stored in the magnetic disc device 4 by the dedicated computation processing device 15 these lsi mask pattern data are read out by the cpu 1 the new lsi mask pattern 60 shown in fig7 being displayed on the graphic display device 2 the drawing of fig7 shows new lower level cells 62 through 64 obtained by compaction processing the lower level cells 32 through 34 as well as a new upper level cell 65 obtained by compaction processing the upper level cell 35 thus lsi mask pattern data can be compaction processed with the hierarchical structure thereof being retained this results in a substantial reduction in the amount of data to be compaction processed.