We examined the mechanism by which contingent negative variation (CNV) amplitude in the prefrontal cortex during the preparatory period of a stop-signal task affected the accuracy of response inhibition in the task. The participants were required to press a button when a go signal was presented and withhold the response when the go signal was followed by a stop signal. Continuous electroencephalograms were recorded of the six electrodes (Fz, F3, F4, Cz, C3, and C4) in the regions of interest during the performance of the task. First, the fast and slow go responses in the preparatory periods were compared. The activities in the preparatory periods of the successful and failed inhibitions were then compared and analyzed. The late CNV amplitudes of F4 and Cz were significantly larger for fast go responses than for slow go responses. Moreover, the late CNV amplitudes of almost all of the channels, with the exception of C3 and C4, were significantly larger for failed inhibitions than for successful inhibitions. These findings suggested that increased prefrontal activity during the preparatory period facilitated the execution process after the presentation of the go signal. Because execution processing is completed more quickly than stop processing, the response inhibition then failed.