1. A disparity deriving apparatus for deriving disparity of an object based on a reference image of the object captured at a first image capturing position and a comparison image of the object captured at a second image capturing position, the disparity deriving apparatus comprising: a calculator to calculate costs between a first reference area in the reference image and each one of corresponding areas corresponding to the first reference area in a given region in the comparison image, and costs between a second reference area, surrounding the first reference area in the reference image, and each one of corresponding areas corresponding to the second reference area in a given region in the comparison image; a synthesizer to synthesize the costs of the first reference area, and the costs of the second reference area as synthesis costs; a deriving unit to derive a disparity value of the object captured in the first reference area based on the synthesis costs synthesized by the synthesizer; and an output unit to output the disparity value derived by the deriving unit and one of the costs of the first reference area that is identified by the derived disparity value.