First, an amount of a current flowing between a first wiring and a third wiring
is estimated, and the number of stack vias required for connecting the first wiring
and the third wiring is determined. Next, based on the number of stack vias, the
number of virtual wirings for determining positions of the stack vias is determined.
Thereafter, the virtual wirings are arranged in a forming region of the third wiring
above the first wiring, for example, at an equal interval, and the stack vias are
created in intersections of the first wiring and the virtual wirings. Thereafter,
the virtual wirings are removed, and the third wiring is created. According to
needs, a second wiring passing between the stack vias is created.