1. A calibration apparatus for calibrating a parameter related to positions of first and second 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: a processing circuit, to receive inputs of two reference images and plural items of parallax data, the two reference images being captured by the first imaging device at two locations, and the parallax data being calculated using the 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, the two corresponding images being captured by the second imaging device at the two locations; search for the plural feature points common to the two reference images; calculate parallaxes and parallax changing amounts thereof, based on the parallax data related to the respective feature points in the two reference images, for each of the searched for feature points; create a model curve using the calculated parallaxes and parallax changing amounts thereof, the model curve being a quadratic curve that expresses a relation between the parallaxes and the parallax changing amounts, wherein the model curve is the quadratic curve in a coordinate system in which a lateral axis is represented by one of the parallaxes and the parallax changing amounts, and a longitudinal axis is represented by the other of the parallaxes and the parallax changing amounts; normalize the parallax changing amounts if the parallax data is evaluated to be reliable; store the normalized parallax changing amounts, wherein the normalized parallax changing amounts are associated with the corresponding parallaxes; determine if the stored normalized parallax changing amounts and the corresponding parallaxes are reliable if a number of the stored normalized parallax changing amounts is greater than a predetermined value; and calculate a correction value for the parameter related to the positions of the imaging devices as a parallax offset by an intercept of the model curve.