A filter design technique using numerical simulation is described and an example is given of the application of our technique for inverters compliant with European standards. We first studied noise generation mechanisms of inverters with a combined method of magnetic field and an electric circuit. We then design X-capacitor filters for category C2 of the European standard using transmission property analysis. Finally, we achieve compliance with category C2 by reducing the inductance of filter wiring on the double X-capacitor filter.