An image processing method comprises obtaining three-dimensional image data representing a three-dimensional image of a cell aggregate with a cavity inside obtained by optical coherence tomography imaging, obtaining a thickness distribution representing a thickness of the cell aggregate between an outer surface of the cell aggregate facing an outside space and an inner surface thereof facing the cavity at each position based on the three-dimensional image data, and detecting protrusion parts having a prominent thickness as compared to a surrounding region based on the thickness distribution.