To provide a circuit board designing method capable of surely reducing unnecessary radiation and reducing also manufacturing cost and a recording medium.

Respective signal lines are virtually wired with a virtual cross section read out from a virtual cross section describing table and its unnecessary radiation quantity X is calculated (S3, S4). As to a signal line of which unnecessary radiation quantity X exceeds an allowable value A, an improved solution N1 obtained by improving the virtual cross section and an improved solution N2 obtained by inserting countermeasure parts are calculated (S5 to S7). Then, both the improved solutions N1, N2 are allocated to respective layer structure parts read out from a layer structure describing table (S8). Then, executable solutions P are extracted from the combinations of the improved solutions N1, N2 and respective layer structure parts and an optimum solution Q is selected out of the extracted solutions P based on manufacturing cost and the unnecessary radiation quantity X (S9, S10). Then, respective signal lines are automatically wired to the layer structure determined by the optimum solution Q (S11).