A disparity value deriving device includes a calculator configured to calculate costs of candidates for a corresponding region in a comparison image that corresponds to a reference region in a reference image, based on luminance values of the regions. The device also includes a changer configured to change a cost exceeding a threshold to a value higher than the threshold; a synthesizer configured to synthesize a cost of a candidate for a corresponding region for one reference region after the change and a cost of a candidate for a corresponding region for another reference region after the change; and a deriving unit configured to derive a disparity value based on a position of the one reference region and a position of the corresponding region in the comparison image for which the cost after the synthesis is smallest.