There is provided a method including generating a phase contrast image of an aggregate of a pluripotent stem cell from a hologram in which the aggregate is captured; deriving, based on the phase contrast image, an index value indicating a complexity or simplicity of a contour line, the index value being determined according to a relationship between an area of the aggregate and a length of the contour line of the aggregate; and sorting the pluripotent stem cell based on the index value.