The relationship between sleepiness and amplitude of amplitude-modulation following response (AMFR) was investigated to find the best tone to raise awareness of drowsy driver. In this study, AMFRs evoked by amplitude modulated (AM) tones whose carrier frequencies were 500, 1000, 4000, and 8000 Hz were recorded in 17 subjects. Sleepiness level was divided into three stages (Wake, Drowsiness, and Sleep) by using reaction time of psychomotor vigilance task. We found that powers of AMFR evoked by 1000, 4000 and 8000 Hz AM tone were decreased as the increment of the sleepiness, but power of the response evoked by 500 Hz AM tone did not change between Wake and Drowsiness. These results suggest that the low frequency tone could make drivers recognize their own sleepiness and help to reduce the risk of drowsy driving.