The invention provides a stereo image processing device for a vehicle. The device detects whether a stereo camera shifts in position when a vehicle is running, thus preventing in advance bad situations of vehicle control caused by false identification due to the position shift. In the lane line detection performed by a stereo image processing part and an identification processing part, changes in brightness in the road width direction in a image plane are evaluated, a point group serving as a lane line candidate is extracted, and a lane line model is generated by calculating the parallax of left and right lane line candidate points and transforming the result into real space coordinates. A camera displacement diagnosis module determines a camera displacement by calculating a diagnostic value based on a difference between the parallaxes of the left and right lane lines in the same horizontal line between reference comparative images captured by a stereo camera unit. Where the camera displacement has occurred, a vehicle control device is instructed to stop the control function or make a transition to failsafe control, and a warning is issued instructing a user to take the vehicle to a maintenance site for inspection.