A stereo image processor (1) performs image matching, in an image matching section (6), on partial images of the same object included in a base image and a comparative image respectively and detects pixel-precision partial displacement between the base image and the comparative image based on a result of the image matching. Next, in an inverted phase filter processing section (9) in a second matching section (8), pixel values from the comparative image are filtered with an inverted phase filter using, as a filter coefficient, pixel values from the base image with the order thereof reversed, given the partial images of the same object that have minimum pixel-precision partial displacement. Then, in a peak detecting section (10), a peak position where an output value of the filtering with the inverted phase filter is maximum is detected and sub-pixel-precision partial displacement between the base image and the comparative image is detected based on the peak position. In this way, there is provided a stereo image processor that has improved accuracy of disparity calculation and analytical resolution and that requires less computation for disparity calculation and is capable of fast processing.