A device determining a biological state of a driver more accurately, including a hypnagogic symptom phenomenon detecting mechanism, an imminent sleep phenomenon detecting mechanism, a subjective sleepiness / low consciousness traveling state detecting mechanism, and a homeostasis function level determining mechanism, which are configured to function in parallel. Therefore, detection of a hypnagogic symptom phenomenon or an imminent sleep phenomenon, or detection of a period resisting a light sleepiness mild sleepiness or a strong sleepiness which occurs consciously, or the case where a low consciousness traveling state due to a decrease in consciousness level momentarily occurs or the case where it occurs longer continuously, or the like, can be determined / detected, and the driver's biological state can be accurately determined