A method of monitoring a driver including obtaining images from the first and second camera unit, checking whether a brightness of the image obtained from the first camera unit is equal to or greater than a reference value or whether the driver is detected from the image obtained from the second camera unit, if the brightness of the image obtained from the first camera unit is not equal to or greater than the reference value or the driver is not detected from the image obtained from the second camera unit, obtaining a first comparative image from the first camera unit by turning off the second illumination unit and a second comparative image from the first camera unit by turning on the second illumination unit, checking whether a difference value between a brightness of the obtained first comparative image and a brightness of the obtained second comparative image is equal to or greater than a reference difference value, and if the difference value between the brightness of the obtained f comparative image and the brightness of the obtained second comparative image is not equal to or greater than the reference difference value, turning off the second illumination unit.