1. A method of analyzing a cause of springback, comprising: performing a forming analysis through a numerical simulation based on a forming condition of a plastically formed product so as to calculate forming data of the formed product; decomposing, over an entire of the formed product, stress data included in the forming data of the formed product into an in-plane stress component and a bending moment component with respect to at least one directional component of directional components of the stress; generating, from the forming data of the formed product, an individual decomposition data including at least one of a first individual decomposition data and a second individual decomposition data as a before-calculation individual decomposition data, the first individual decomposition data having only an in-plane stress component regarding the stress of the decomposed directional component, the second individual decomposition data having only a bending moment component regarding a stress of the decomposed directional component; performing a calculation for at least one directional component of stress in the before-calculation individual decomposition forming data regarding each of areas divided from the formed product, so as to generate an after-calculation individual decomposition forming data; analyzing a first springback configuration obtained through a numerical simulation with respect to the before-calculation individual decomposition forming data and a second springback configuration obtained through a numerical simulation with respect to the after-calculation individual decomposition forming data; obtaining a degree of influence of a stress in each of the areas with respect to a springback deformation, calculated based on a before-springback configuration of the formed product included in the forming data, the first springback configuration, and the second springback configuration; and displaying the degree of influence with respect to the springback deformation calculated for each area.