The effects of the subthreshold visual stimuli on the electrical activity of the brain were investigated by an oddball-like paradigm and a single-stimulus paradigm. In the oddball-like paradigm, suprathreshold and subthreshold stimuli were presented randomly where the suprathreshold stimulus probability was 0.24. The single-stimulus paradigm contained only the suprathreshold stimuli and was identical to the oddball-like paradigm apart from the absence of subthreshold stimuli. Suprathreshold P3b amplitude was larger for the oddball-like than for the single-stimulus condition particularly in the centro-parietal region. This result suggests that at least one component of the P300 wave could be assigned to an automatic comparison and memory updating process which runs unconsciously.