PURPOSE: To shorten a critical path and to set the number of nets crossing a cut line to a minimum.

CONSTITUTION: A critical path extraction device 101 extracting the critical path, a cut line generation device 103 generating the cut line, a merging opponent selection device 102 deciding the merging opponents of respective blocks from the connection degree of the respective blocks and critical path information, a merging device 104 merging the blocks from the merging opponents of the respective blocks, which are obtained by the merging opponent selection device, and a pairwise device 105 exchanging a pair so that the number of the nets crossing the cut line becomes the minimum are provided.