In this study, we specifically considered decrease in arousal level stemming from using semi-autonomous driver assistances featuring ACC and LKA functions in operation. 10 young men took part in the experimental study using a driving simulator. For an average of 10 people, the participant rate for those who reached level 4 or above (level 1; Seems not sleepy at all, level 5: Very sleepy) on the sleepiness scale when the system was enabled increased by 34 percent compared with the system-disabled condition. The reaction time to visual stimuli was noticeably delayed and significant differences are acknowledged for a level 4 or above on the sleepiness rating scale. We believe that continuous long term investigation on actual driving (FOT; field operation test) should be carried out to validate the results in this study. We also analysed the relationship between eye closure ratio and the sleepiness rating scale and confirmed that eye closure ratio value of 0.26 corresponded to level 4 on the sleepiness scale where the reaction time delayed remarkably. Drawing on these findings, it is believed that a device can be designed to detect arousal level based on eye closure rate.