1. An ophthalmologic apparatus that measures, by light interference, a dimension of an eye to be examined, the ophthalmologic apparatus comprising: a light source; an incidence unit that causes light from the light source to be incident on a plurality of different positions in the eye to be examined; an acquisition unit that acquires a two-dimensional tomographic image of an interior of the eye to be examined on the basis of a plurality of interference signals acquired as a result of the incidence unit causing the incidence of light on the plurality of different positions; a first calculation unit that calculates an axial length value of a measuring portion inside the eye to be examined by determining positions of a front surface and a rear surface of the measuring portion on the basis of the plurality of interference signals; a display unit that displays the acquired two-dimensional tomographic image; and a correction unit that corrects the axial length value of the measuring portion from the two-dimensional tomographic image displayed by the display unit; wherein the display unit displays the positions of the front surface and the rear surface determined by the first calculation unit with the acquired two-dimensional tomographic image, and the correction unit comprises: a first designation unit that designates a new position for at least one of the positions of the front surface and the rear surface with respect to the two-dimensional tomographic image displayed by the display unit; and a second calculation unit that recalculates the axial length value on the basis of the new position designated by the first designation unit.