A computer-implemented image processing method for an image of a cell aggregate with a cavity inside as a processing object, the image processing method comprising: obtaining three-dimensional image data representing a three-dimensional image of an embryo as the cell aggregate 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 corresponding to individual cells constituting a trophectoderm and having a prominent thickness as compared to a surrounding region based on the thickness distribution, wherein: polar coordinate data representing a position of each point of the cell aggregate as a position in a polar coordinate space are generated from three-dimensional image data; an origin of the polar coordinate space is set to a position of a center of gravity of the cell aggregate; the thickness distribution of the cell aggregate is obtained based on the thickness of the cell aggregate obtained along a radius from the origin; and the protrusion part is detected based on a thickness profile of the cell aggregate associating a radial direction in the polar coordinate space and the thickness of the cell aggregate obtained in this radial direction, wherein: a maximum value filter processing is applied to the thickness profile; and a position where there is no change in value before and after the maximum value filter processing is detected as the position of the protrusion part. obtaining three-dimensional image data representing a three-dimensional image of an embryo as the cell aggregate 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 corresponding to individual cells constituting a trophectoderm and having a prominent thickness as compared to a surrounding region based on the thickness distribution, wherein: polar coordinate data representing a position of each point of the cell aggregate as a position in a polar coordinate space are generated from three-dimensional image data; an origin of the polar coordinate space is set to a position of a center of gravity of the cell aggregate; the thickness distribution of the cell aggregate is obtained based on the thickness of the cell aggregate obtained along a radius from the origin; and the protrusion part is detected based on a thickness profile of the cell aggregate associating a radial direction in the polar coordinate space and the thickness of the cell aggregate obtained in this radial direction, polar coordinate data representing a position of each point of the cell aggregate as a position in a polar coordinate space are generated from three-dimensional image data; an origin of the polar coordinate space is set to a position of a center of gravity of the cell aggregate; the thickness distribution of the cell aggregate is obtained based on the thickness of the cell aggregate obtained along a radius from the origin; and the protrusion part is detected based on a thickness profile of the cell aggregate associating a radial direction in the polar coordinate space and the thickness of the cell aggregate obtained in this radial direction, a maximum value filter processing is applied to the thickness profile; and a position where there is no change in value before and after the maximum value filter processing is detected as the position of the protrusion part.