Layout information indicating locations of at least components and conductive layers in a printed circuit board, and layouts of conductive wiring patterns on the respective conductive layers and vias which electrically connect between the conductive layers is obtained from a memory. With reference to the layout information, path information indicating a path of one signal line is generated. With reference to the layout information and path information, a divide portion where a path of a return current corresponding to a signal current of the signal line is divided are detected. With reference to the layout information and path information, information indicating a detour path of the return current in a neighborhood of the divide portion is generated.