1. A method of generating a motion contrast image using optical coherence tomography, comprising: a data obtaining step of obtaining a plurality of pieces of tomographic image data each indicating a tomogram at substantially the same portion of an object; a removing step of removing an outlier from the plurality of pieces of tomographic image data; a computation step of computing a motion contrast value based on the plurality of pieces of tomographic image data from which the outlier is removed; and a generation step of generating a motion contrast image of the object based on the motion contrast value.