1. A face detection system for a vehicle, comprising: at least one first lighting unit for radiating infrared light onto a left side of a driver's face; at least one second lighting unit for radiating infrared light onto a right side of the driver's face an image capturing unit for separately capturing the driver's face onto which the infrared light is radiated from the first lighting unit or units and the second lighting unit or units; and a control unit for acquiring left and right images of the face from the image capturing unit, and obtaining a difference image between the acquired left and right images, thus determining whether the driver is inattentive in looking ahead.