An image reducer executes a reduction process on a comparison image to generate a reduced comparison image, and executes a reduction process on a reference image to generate a reduced reference image. A disparity value deriver (a deriving unit) derives a disparity value by setting the minimum value of synthesized costs of pixels in the reduced comparison image in connection with a reference pixel in the reduced reference image, which have been calculated by a cost synthesizer. A disparity image generator generates a reduced disparity image, which is an image obtained by replacing the luminance values of the respective pixels in the reduced reference image with the disparity values corresponding to the pixels. The disparity image enlarger executes an enlargement process for enlarging the image size of the reduced disparity image generated by the disparity image generator to generate a disparity image.