A calibration apparatus for calibrating a parameter related to positions of cameras used as two imaging devices (20, 21), the cameras having the same focal length and parallel optical axes and 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 (20, 21) at two locations during a straight line movement of the mobile object, 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 for each of the locations, these two corresponding images being captured by another of the imaging devices (20, 21) at these two locations during the straight line movement of the mobile object; 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; a correction value calculating part configured to calculate a correction value for the parameter related to the positions of the imaging devices (20, 21) based on the calculated parallaxes and parallax changing amounts thereof, and to create a model curve indicating a relationship between the calculated parallaxes and parallax changing amounts thereof and calculate the correction value based on the model curve; and a reliability evaluating part configured to evaluate the presence or absence of reliability of the correction value based on a distribution of the parallaxes and dispersion of the parallax changing amounts with respect to the model curve, characterized in that the reliability evaluating part extracts only the data of the parallax changing amounts which are within a predetermined range from the parallax changing amounts on the model curve; calculates an average root-mean-square of differentials between the parallax changing amounts of the extracted data and the parallax changing amounts for the corresponding parallaxes on the model curve; and determines whether the calculated average root- mean-square is smaller than or equal to a threshold. 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 (20, 21) at two locations during a straight line movement of the mobile object, 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 for each of the locations, these two corresponding images being captured by another of the imaging devices (20, 21) at these two locations during the straight line movement of the mobile object; 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; a correction value calculating part configured to calculate a correction value for the parameter related to the positions of the imaging devices (20, 21) based on the calculated parallaxes and parallax changing amounts thereof, and to create a model curve indicating a relationship between the calculated parallaxes and parallax changing amounts thereof and calculate the correction value based on the model curve; and a reliability evaluating part configured to evaluate the presence or absence of reliability of the correction value based on a distribution of the parallaxes and dispersion of the parallax changing amounts with respect to the model curve, characterized in that the reliability evaluating part extracts only the data of the parallax changing amounts which are within a predetermined range from the parallax changing amounts on the model curve; calculates an average root-mean-square of differentials between the parallax changing amounts of the extracted data and the parallax changing amounts for the corresponding parallaxes on the model curve; and determines whether the calculated average root- mean-square is smaller than or equal to a threshold.