The present invention relates to a face detection system for a vehicle. At least one first lighting unit is configured to radiate infrared light onto a left side of a driver's face. At least one second lighting unit is configured to radiate infrared light onto a right side of the driver's face. An image capturing unit separately captures the driver's face onto which the infrared light is radiated from the first and second lighting units. A control unit acquires left and right images of the face from the image capturing unit, and obtains a difference image between the acquired left and right images, thus determining whether the driver is inattentive in looking ahead. The system stably performs the face detection function with no or less effect by external optical environments as well as reduced computational load.