Standard cells without a well potential fixing active region (4T-11 to 4T-14, 4T-21 to 4T-24, 4T-31 to 4T-34, 4T-41 to 4T-44) are read from a library and a circuit is temporarily designed by automatic layout wiring. Then, a change in the substrate potential is estimated from at least one of the number of transistors to be switched at the same timing in the temporarily designed circuit, the sizes of transistors, the transition probability, and the appearance probability. It is determined whether the estimated change in the substrate potential is within a reference value. If the estimated change in the substrate potential has exceeded the reference value, standard cells with a well potential fixing active region (2T-11, 2T-21, 2T-31 and 2T-41) are read from the library and placed in a region where the estimated change in the substrate potential exceeds the reference value. Thereafter, automatic layout wiring is done again, thereby forming a circuit.