1. A stereo image processing apparatus that computes a shift between a target image and a reference image that form stereo images, the stereo image processing apparatus comprising: an extraction section that extracts M many divisional target images obtained by dividing a first partial region in the target image into N parts (where N is a natural number equal to or greater than 2, and M is a natural number equal to or greater than 2 but equal to or less than N), and extracts M many divisional reference images obtained by dividing a second partial region in the reference image corresponding to the first partial region in the target image into N parts; a concatenating section that forms a first concatenated data sequence by concatenating, in series, M many data sequences, each of which comprises intensity values of a corresponding one of the divisional target images, and forms a second concatenated data sequence by concatenating, in series, M many data sequences, each of which comprises intensity values of a corresponding one of the divisional reference images; a filtering section that computes inverted phase filter coefficients by reversing the data order of the first concatenated data sequence, and performs filtering on the second concatenated data sequence using the inverted phase filter coefficients; and a computation section that computes the shift based on a peak position in a filtering result of the filtering section.