Event Related Potential (ERP) of brain EEG (Electroencephalogram) activity plays an important role in EEG phase synchronization and/or hemodynamic responses measured by fMRI (functional Magnetic Resonance Imaging). However, the specific mechanism of ERP generation is still unclear. In this study, pulse and noise type visual stimuli are administered to subjects. Then the phase response of the EEG in α-waves is analyzed. As a result, the magnitudes of the phase response are varied by the stimulus administered phase and no power increase is observed. These results indicate that the ERP in α-waves is generated by the phase resetting of brain activity.