To judge the mental fatigue levels of individual drivers even in a vehicle used by the plural drivers and to accurately judge the mental fatigue level generated by driving work.

The driver presses the touch panel switch of a driver specifying device 1 and inputs driver information and a traveling condition detector 2 detects the traveling condition of the vehicle and outputs the traveling mode of 'stop mode', 'low speed mode' or 'high speed mode'. The electrocardiogram signals of the driver are measured in an electrocardiogram signal detector 3 and a sympathetic and parasympathetic nerve activity level calculation circuit 4 obtains RRI data which are a beat interval from the electrocardiogram signals for the respective traveling modes, eliminates defective data including detection errors and noise, calculates the level of the dispersion of the RRI data and an average heart rate and obtains a centroid position on a two-dimensional plane. In a mental fatigue level judgement circuit 5, the measured centroid position and the average centroid position in the same traveling mode of the same driver as the measured driver stored in a memory 6 are compared and a mental fatigue level is judged.