An estimated value of wakefulness is calculated from the ratio between the quantity of high frequency components and the quantity of low frequency components obtained by performing frequency transformation of vehicle displacements (S51). An imaginary deviation line is set between a lane marking and the vehicle according to the estimated value of wakefulness (S55, S59, S62, S66), and an alarm is issued when the vehicle deviates from the imaginary deviation line. Since the alarm is issued when the vehicle is at a predetermined distance from the lane marking, alarming can be performed at substantially constant timing.