A net detecting unit detects a set of component terminal interconnection information showing a critical net from a component terminal interconnection information list. A conductor detecting unit detects a conductor corresponding to the critical net. A component detecting unit detects two components from the set of component terminal interconnection information. A terminal detecting unit detects a power and/or ground terminal of each of the detected components. A power/ground layer detecting unit detects at least one layer, among power and ground layers, to which the detected power and/or ground terminals are connected. A layer detecting unit specifies a layer, among the detected layers, that is nearest to a signal layer on which the conductor is placed. A prohibition area generating unit generates a via prohibition area on the specified layer. As a result, vias are placed on the specified layer, avoiding the via prohibition area.