An ophthalmologic apparatus comprising: a fixation projection system configured to project fixation light flux onto a fundus of a subject's eye; an interference optical system including an optical scanner and configured to split light from light source into measurement light and reference light, to irradiate the subject's eye fundus (Ef) with the measurement light deflected by the optical scanner, and to detect interference light between returning light of the measurement light from the subject's eye fundus (Ef) and the reference light; a controller configured to perform OCT measurement on a first scan region and a second scan region, which are different from each other in the subject's eye fundus (Ef), by controlling the interference optical system in a state where a projected position of the fixation light flux on the fundus is fixed by fixing a position at which a fixation target is presented to the subject's eye fundus (Ef); an image forming unit configured to form an image of the subject's eye fundus (Ef) based on detection result of the interference light; an image composing unit configured to compose a first image formed by the image forming unit and a second image formed by the image forming unit, the first image being formed based on detection result of the interference light obtained by performing OCT measurement on the first scan region, the second image being formed based on detection result of the interference light obtained by performing OCT measurement on the second scan region; characterised by: an optical path length difference changing unit configured to change a difference of optical path lengths between an optical path of the measurement light and an optical path length of the reference light, wherein the controller is configured to change the difference of the optical path lengths by controlling the optical path length difference changing unit in accordance with a position of the first or second scan region in the subject's eye fundus (Ef), and to change a reference position of a measurement range in a depth direction of the OCT measurement on the subject's eye fundus (Ef). a fixation projection system configured to project fixation light flux onto a fundus of a subject's eye; an interference optical system including an optical scanner and configured to split light from light source into measurement light and reference light, to irradiate the subject's eye fundus (Ef) with the measurement light deflected by the optical scanner, and to detect interference light between returning light of the measurement light from the subject's eye fundus (Ef) and the reference light; a controller configured to perform OCT measurement on a first scan region and a second scan region, which are different from each other in the subject's eye fundus (Ef), by controlling the interference optical system in a state where a projected position of the fixation light flux on the fundus is fixed by fixing a position at which a fixation target is presented to the subject's eye fundus (Ef); an image forming unit configured to form an image of the subject's eye fundus (Ef) based on detection result of the interference light; an image composing unit configured to compose a first image formed by the image forming unit and a second image formed by the image forming unit, the first image being formed based on detection result of the interference light obtained by performing OCT measurement on the first scan region, the second image being formed based on detection result of the interference light obtained by performing OCT measurement on the second scan region; characterised by: an optical path length difference changing unit configured to change a difference of optical path lengths between an optical path of the measurement light and an optical path length of the reference light, wherein the controller is configured to change the difference of the optical path lengths by controlling the optical path length difference changing unit in accordance with a position of the first or second scan region in the subject's eye fundus (Ef), and to change a reference position of a measurement range in a depth direction of the OCT measurement on the subject's eye fundus (Ef). an optical path length difference changing unit configured to change a difference of optical path lengths between an optical path of the measurement light and an optical path length of the reference light, wherein the controller is configured to change the difference of the optical path lengths by controlling the optical path length difference changing unit in accordance with a position of the first or second scan region in the subject's eye fundus (Ef), and to change a reference position of a measurement range in a depth direction of the OCT measurement on the subject's eye fundus (Ef).