1. An image processing system, comprising: a memory that stores a plurality of tomographic images acquired based on a measuring beam controlled so that a same position is scanned with the measuring beam a plurality of times; and a processor configured to calculate a degree of similarity among partial areas which are respectively included in the plurality of tomographic images stored in the memory, configured to generate a composite image by combining a plurality of tomographic images having a calculated degree of similarity being larger than a predetermined value, wherein the plurality of tomographic images are acquired via a Fourier-domain optical coherence tomography apparatus, and the processor calculates the degree of similarity among the partial areas according to translation or rotation of the plurality of tomographic images.