The aim of this study was to identify a useful measure to estimate an arousal level of drivers, to apply the result to develop ITS (Intelligent Transportation System) that can warn drivers of their low arousal state and to prevent driving under low arousal level from occurring and contribute to the reduction of traffic accidents. The EEG (electroencephalography) and ECG(electrocardiography) during a monotonous task was measured, and it was investigated how these measures change under the low arousal (drowsy) state. The time series of mean power frequency of EEG was plotted on X-bar control chart. Heart rate variability (HRV) measures RRV3 and RRV8-3 were derived on the basis of R-R intervals (interbeat intervals) obtained from ECG. Under the low arousal state (drowsy state), the mean power frequency tended to be lower than central line (CL) and range between CL and lower control limit (LCL). In accordance with this, RRV3 and RRV8-3 tended to increase under the low arousal (drowsy) state, which means that the parasympathetic nervous system became dominant under drowsy states.