When a steering angle dakaku is equal to or smaller than .+-.15.degree. (S22), the steering angle dakaku is integrated to calculate an integrated value dakaku_sum (S24). When the integrated value dakaku_sum is within a set range (S25, S26), a road-shape-based wakefulness correction coefficient R_hosei is set at 0.5 (S28). Any erroneous determination of wakefulness attributable to a road shape is prevented by correcting wakefulness H for evaluating wakefulness by decreasing it using the road-shape-based wakefulness correction coefficient R_hosei.