1. A calibration apparatus comprising: distance image generation circuitry which, in operation, calculates a distance from a stereo camera for each of a plurality of pixels included in a stereo image including a road surface and that generates a distance image in which each of the plurality of pixels is associated with the calculated distance; first region detection circuitry which, in operation, detects a road surface image region based on the stereo image including the road surface; second region detection circuitry which, in operation, detects a setting prohibited region including a group of non-effective pixels with which calculating the distance in the distance image is not possible; area setting processing circuitry which, in operation, adjusts an arrangement pattern in a direction in which positions of a plurality of area setting frames become distant from each other, based on the road surface image region and the setting prohibited region; extraction processing that circuitry which, in operation, extracts, from the distance image, a plurality of partial images corresponding to the plurality of area setting frames included in the adjusted arrangement pattern; and calculation circuitry which, in operation, calculates an installation state parameter used in an installation state of the stereo camera of the stereo image, based on the plurality of extracted partial images.