Method of designing circuit board

The method of designing a circuit board, comprising:

• setting, with respect to a circuit board, mounting positions of a plurality of electronic devices positions of pads to which terminals of the electronic devices are connected, and positions of external connection terminals;
• forming a rats nest by connecting said pads and external connection terminals by lines and connecting pads to which terminals of the plurality of electronic devices are connected by lines;
• extracting a region with the highest density of lines of the rats nest;
• setting design rules relating to routes and dimensions of conductor lines in the region with the highest density of lines of the rats nest;
• laying conductor lines at the region with the highest density of lines of the rats nest;
• confirming whether or not the conductor lines can be laid at the region with the highest density of lines of the rats nest;
• repeating setting of the design rules and laying of the conductor lines if the conductor lines cannot be laid, and
• laying conductor lines of the remaining regions by the set design rules if the conductor lines can be laid.