To improve the stereo image processing accuracy of a stereo camera by eliminating the occurrence of a luminance offset between pictures in the camera.

Luminance histograms are respectively prepared with respect to both left and right images (S1) and the horizontal deviation of each histogram of the left and right images is found as a luminance deviation value Eb (S2). Then the deviation value Eb is converted into the correction amount Do of the clamp value to a clamp circuit (S3) and a directive value obtained by adding the correction amount Do to the present clamp value is outputted to the clamping circuit (S4). Consequently, the luminance levels of the left and right images are made even by adjusting the clamp level for A/D conversion and the stereo image processing accuracy of the stereo camera can be improved by automatically eliminating the luminance offset between images which has been difficult to be eliminated by the conventional gain adjustment.