In a vehicle night vision system, the display unit is activated only when an illumination level below the prescribed level has persisted for more than the prescribed period of time so that the display unit would not be activated when the low illumination level lasts only for a short period of time and the night vision system would not be required. Therefore, when the vehicle passes through a short tunnel during daytime, the display unit would not be activated and the vehicle operator is prevented from being unnecessarily distracted. On the other hand, when the vehicle passes through a long tunnel or when the sun has set and the illumination level of the road ahead is low, the night vision system is activated without fail, and assists the vehicle operator's vision.