A computer-implemented method for optimizing a portion of a structural body model including a movable portion of an automotive body, the movable portion relating to a part including a steel sheet, whereby the optimizing realizes weight reduction of a structural body while improving stiffness or crash worthiness, by using three-dimensional elements, the method comprising: a design space setting step of setting, as a design space, the portion to be optimized in the movable portion; an optimization block model generating step of generating, in the set design space, an optimization block model in the portion, the optimization block model being formed of three-dimensional elements and to be subjected to optimization analysis processing; a connection processing step of connecting the generated optimization block model with the structural body model so that a load is transmitted through the connecting; a material property setting step of setting a material property for the optimization block model; an optimization analysis condition setting step of setting an optimization analysis condition for finding an optimum shape of the optimization block model, the optimization analysis condition comprising an objective condition including one of minimizing displacement, minimizing strain energy, minimizing generated stress, or maximizing absorbed energy, and a constraint condition including at least one of a material volume fraction, displacement of an arbitrary portion, a generated stress; a multi-body dynamics analysis condition setting step of setting a multi-body dynamics analysis condition for performing multi-body dynamics analysis on the structural body model with which the optimization block model has been connected; and an optimum shape analyzing step of executing, based on the set optimization analysis condition and multi-body dynamics analysis condition, the multi-body dynamics analysis on the optimization block model and thereby finding the optimum shape of the optimization block model in the portion. a design space setting step of setting, as a design space, the portion to be optimized in the movable portion; an optimization block model generating step of generating, in the set design space, an optimization block model in the portion, the optimization block model being formed of three-dimensional elements and to be subjected to optimization analysis processing; a connection processing step of connecting the generated optimization block model with the structural body model so that a load is transmitted through the connecting; a material property setting step of setting a material property for the optimization block model; an optimization analysis condition setting step of setting an optimization analysis condition for finding an optimum shape of the optimization block model, the optimization analysis condition comprising an objective condition including one of minimizing displacement, minimizing strain energy, minimizing generated stress, or maximizing absorbed energy, and a constraint condition including at least one of a material volume fraction, displacement of an arbitrary portion, a generated stress; an objective condition including one of minimizing displacement, minimizing strain energy, minimizing generated stress, or maximizing absorbed energy, and a constraint condition including at least one of a material volume fraction, displacement of an arbitrary portion, a generated stress; a multi-body dynamics analysis condition setting step of setting a multi-body dynamics analysis condition for performing multi-body dynamics analysis on the structural body model with which the optimization block model has been connected; and an optimum shape analyzing step of executing, based on the set optimization analysis condition and multi-body dynamics analysis condition, the multi-body dynamics analysis on the optimization block model and thereby finding the optimum shape of the optimization block model in the portion.