A vehicle body assembling method for forming an inner frame of a vehicle body by using a floor assembly, side inner assemblies, and a roof arch, comprising: a first holding step for holding the floor assembly by a first jig in a state in which the floor assembly is positioned at a predetermined holding position; a second holding step for holding the roof arch by a second jig in a state in which the roof arch is positioned at a predetermined holding position; a jig connecting step for connecting the first jig and the second jig so as to mutually position the floor assembly and the roof arch after the first holding step and the second holding step; a third holding step for holding the side inner assemblies by the first jig and the second jig in a state, in which the side inner assemblies are partly in contact with and positioned with respect to the floor assembly and the roof arch, after the jig connecting step; and a welding step for connecting the floor assembly, the roof arch, and the side inner assemblies by welding after the third holding step, thereby forming the inner frame; the vehicle body assembling method being characterized in that the welding step comprises: a first welding step for carrying out welding at some of all welding positions to preliminarily connect the floor assembly, the roof arch, and the side inner assemblies; a second jig retraction step for clearing the holding of the roof arch and the side inner assemblies by the second jig so as to retract the second jig to a predetermined position after the first welding step; and a second welding step for carrying out welding at remaining welding positions after the second jig retraction step; wherein, after the first welding step, the connection between the first jig and the second jig is partly cleared before the second welding step. a first holding step for holding the floor assembly by a first jig in a state in which the floor assembly is positioned at a predetermined holding position; a second holding step for holding the roof arch by a second jig in a state in which the roof arch is positioned at a predetermined holding position; a jig connecting step for connecting the first jig and the second jig so as to mutually position the floor assembly and the roof arch after the first holding step and the second holding step; a third holding step for holding the side inner assemblies by the first jig and the second jig in a state, in which the side inner assemblies are partly in contact with and positioned with respect to the floor assembly and the roof arch, after the jig connecting step; and a welding step for connecting the floor assembly, the roof arch, and the side inner assemblies by welding after the third holding step, thereby forming the inner frame; the vehicle body assembling method being characterized in that the welding step comprises: a first welding step for carrying out welding at some of all welding positions to preliminarily connect the floor assembly, the roof arch, and the side inner assemblies; a second jig retraction step for clearing the holding of the roof arch and the side inner assemblies by the second jig so as to retract the second jig to a predetermined position after the first welding step; and a second welding step for carrying out welding at remaining welding positions after the second jig retraction step; a first welding step for carrying out welding at some of all welding positions to preliminarily connect the floor assembly, the roof arch, and the side inner assemblies; a second jig retraction step for clearing the holding of the roof arch and the side inner assemblies by the second jig so as to retract the second jig to a predetermined position after the first welding step; and a second welding step for carrying out welding at remaining welding positions after the second jig retraction step; wherein, after the first welding step, the connection between the first jig and the second jig is partly cleared before the second welding step.