Is proposed an approach to know magnitudes of parasitic electromagnetic modes excited by the micro-strip line passing over a slit in the return plane. A part of the power having been carried along, the microstrip-line is transferred to the coplanar-line modes along the slot, and the cylindrical. mode in between the pair of re. turn and ground planes. Magnitudes of those modes are expressed by the ratio of powers of the excited electromagnetic mode to the original microstrip line mode, discarding information of phase. Calculation was done with FDTD in the limited domain., around the cross-point in a moderate time of calculation. This approach contributes to the efficient designing of PCB of commercial electronic devices.