To perform reliable fail safe by setting a 2nd monitoring area in one image area while made to positionally correspond to a 1st monitoring area in the other image area.

The luminance value of each pixel in a monitoring area is read to check the luminance state of a stereo image pair (step 1), and whether the difference between the luminance average value A1 of a 1st monitoring area and the luminance average value A2 of a 2nd monitoring area is equal to or greater than a prescribed decision value is decided (step 3). In a normal situation, a fail flag NG is set to 0 on the decision results (step 4). Meanwhile, in an abnormal situation in which brightness unbalance takes place due to disturbance, the fail flag NG is set to 1 (step 5). Thus, fail safe that temporarily stops normal monitoring control is carried out.