Recent years have seen many studies of the detection and the evaluation of drivers' drowsiness. Various systems to detect drowsy driving have also been installed in vehicles. Detecting drivers' drowsiness only is not enough to keep them driving safely. We also need ways of stimulating the drivers' vigilance (by delaying the creeping drowsiness). This paper describes a method where we propose combining steering wheel vibration with the auditory stimulus for the drowsy drivers to raise their vigilance. We conducted experiments on raising driver vigilance as understood from the physiological signal, using a driving simulator. The ratio of alpha-wave in EEG was decreased because of the stimulus given that combines steering wheel vibration with sound. Consequently, the progress of driver's drowsiness was delayed by about 1.7 times compared with the case when no stimulus was given. Combined stimuli were also found to be more effective than that of an individual stimulus. We also observed that synchronizing the sound and vibration stimuli is more effective in raising driver vigilance as compared to a Time-shifted joint mode of delivery. Moreover, deviation of the car from the lane center decreased the least by using combined stimuli in a synchronous mode.