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: processing circuitry configured 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; synthesize the costs of the first reference area, and the costs of the second reference area as synthesis costs; derive a disparity value of the object captured in the first reference area based on the synthesis costs; determine, from the costs of the first reference area, a specific cost that is identified by the derived disparity value; and output the derived disparity value and the determined specific cost that is identified by the derived disparity value.