1. A joint analyzing method, comprising: performing a data measurement in which measurement of data on one or more of stress, strain, and deformation of a joint in each of a load non-applied state and a load applied state is performed, the joint joining two or more members together by at least one junction, the load non-applied state being a state in which an external load is unapplied to the joint, and the load applied state being a state in which the external load is applied to the joint; performing a detailed analysis in which a finite element method analysis is performed using a first analysis model in which geometry of a joint surrounding region is modeled; performing a simplified analysis in which a finite element method analysis is performed using a second analysis model in which the at least one junction is eliminated or simplified and geometry of the joint surrounding region is rougher in element division than the geometry in the first analysis model in the detailed analysis; performing a first repetition analysis in which the detailed analysis is repeated while modifying the first analysis model and an analysis technique of the finite element method in the detailed analysis, until a measurement result of the data measurement and an analysis result of the detailed analysis are brought into agreement with each other within a first predetermined error range, the first repetition analysis obtaining an initial internal load of the at least one junction in the load non-applied state and allowing the simplified analysis to take into consideration the initial internal load; and performing a second repetition analysis in which the simplified analysis that takes into consideration the initial internal load is repeated while modifying the second analysis model and an analysis technique of the finite element method in the simplified analysis, until the measurement result of the data measurement and an analysis result of the simplified analysis are brought into agreement with each other within a second predetermined error range.