Topology optimization, a simulation-based optimization technique to mathematically derive an optimal structure, is applied to the conductor design of an inductance cancellation structure to reduce common-and differential-mode noise. The design of an appropriate conductor pattern for an inductance cancellation structure is difficult due to the unintentional magnetic couplings between the multiple loops mounted to cancel the parasitic inductances of X and Y capacitors. In topology optimization, this design problem is formulated as an optimization problem and an optimal structure is obtained by repeated modification of the conductor pattern using mathematical programming. The performance of circuit boards before and after optimization is evaluated using mixed-mode s-parameters. Both simulation and experimental measurement verify that the effect of inductance cancellation is improved by this optimization.