1. A calibration apparatus for calibrating a parameter related to positions of two imaging devices, the parameter being set in a distance measurement system which is installed in a mobile object and performs distance measurement with the imaging devices, the calibration apparatus comprising: an image obtaining part configured to receive inputs of two reference images and plural items of parallax data, these two reference images being captured by one of the imaging devices at two locations, and the parallax data being calculated using these two reference images and two corresponding images based on positions of plural feature points common to the reference image and the corresponding image for each pair thereof on a location basis, these two corresponding images being captured by another of the imaging devices at these two locations; a feature point searching part configured to search for the plural feature points common to these two reference images; a parallax calculating part configured to calculate parallaxes and parallax changing amounts thereof, based on the parallax data related to the respective feature points in these two reference images, for each of the searched for feature points: and a correction value calculating part configured to calculate a correction value for the parameter related to the positions of the imaging devices based on the calculated parallaxes and parallax changing amounts thereof.