The emergence of autonomous driving is becoming a reality worldwide. However, in emergency situations, human drivers must assume control of the vehicle, hence detecting driver vigilance is critical. Although many electroencephalographic studies have documented alterations in brain rhythms associated with fatigue, the results have been inconsistent. Furthermore, in high-level automated driving contexts, the driver does not operate the vehicle except in emergencies. In this study, magnetoencephalograms from eight healthy participants were recorded as they watched a 20-minute video from the driver’s perspective. The participants were instructed to press a button when a checkered flag appeared, simulating their response to an emergency. Statistical analysis demonstrated that response time to the checkered flag was prolonged during the End period (final third) of the experiment (p = 0.04). The increase in reaction time from the Beginning (first third) to the Middle (middle third) period correlated negatively with alpha-band activities in the occipital cortices during all three periods (Beginning, r = −0.72; p = 0.04; Middle, r = −0.73, p = 0.04; End r = −0.78, p = 0.02). In other words, participants with higher alpha-band activity in the Beginning period did not experience prolonged reaction time toward the Middle period. This result suggests that alpha-band activity in the occipital cortex may serve as an indicator of whether participants maintain vigilance during autonomous driving.