Reduction in arousal is one of the main causes that lead to lapses of driver's attention. Therefore, in order to keep a safe drive by the operation of driver's own, it is important to detect the reduced arousal as soon as possible. Currently we focused on saccadic eye movements, which can be measured noninvasively, as a possible detector of driver's reduced arousal. In this paper, we evaluated saccades while subjects were performing a simple reaction test and a driving simulation. We found that changes in characteristics of horizontal saccades, i. e. peak velocities and inter-saccadic intervals, were synchronized with the fluctuation of pupil area that is a well known objective measure of arousal level. Further, when the pupil area decreased because of reduced arousal, significant reductions of these saccade parameters were found prior to the occurrence of slow eye movement (SEM) that is another indicator of unconsciousness or aimlessness. Based upon these findings, we conclude that slowed peak horizontal saccade velocity (<40 deg/s) and shortened inter-saccadic interval (<0.2 s) are useful detector of driver's reduced arousal.