In this study we investigated the effect of the single pulse transcranial magnetic stimulation (TMS) and repetitive TMS (rTMS) to the supramarginal gyrus (SMG) on an event related potential (ERP) which is called P300. By applying the Oddball task which gives 2 easily distinguishable types of stimuli, the P300, one of the ERP can be observed. Former studies reported that left SMG is related to the origin of P300. In Experiment 1, TMS is applied to the SMG after 150ms, 200ms, and 250ms from presentation of audio-stimuli. By comparing the P-values, the effect of the TMS applied to the left SMG was larger. In Experiment 2, rTMS (100 stimuli with 1Hz) is applied to the SMG. We measured the P300 response before and after rTMS. By comparing the latency and amplitude of P300, latency was confirmed to increase when applied to the left SMG. However, there was no difference of amplitude of P300.