Provided is a stereo image processing apparatus wherein parallax can be calculated with high precision. A window-function shifting unit (411) sets a third window function, which is formed by shifting a second window function on the basis of the amount of deviation in sub-pixel units, onto an image cutting-out unit (412). The image cutting-out unit (412) applies the second window function to a position subjected to a cut-out, cuts out a unit partial target image from a standard image, applies the second window function or the third window function, and cuts out a unit partial reference image from a reference image. A peak-position detection unit (106) calculates the amount of deviation in sub-pixel units on the basis of the phase difference between a data string comprising brightness of the cut-out unit partial target image, and a data string comprising brightness of the cut-out unit partial reference image.