Summary: For emission from a printed circuit board (PCB) by the common-mode current, the suppression method based on the image theory by placing a conducting plate under the PCB is presented. In order to evaluate the suppression effect by this method the radiation power from the PCB is calculated by using FDTD method. The numerical results show that placing the conducting plate suppresses the emission by the common-mode current. Especially, using the conducting plate bent the sides, it is possible to suppress the emission by the small conducting plate. Further, the experimental results of a radiation power and a maximum electric field intensity show the validity of the numerical results.