A battery module manufacturing system, comprising: a battery module including a cell stack formed by stacking a plurality of battery cells, a module case accommodating the cell stack, a bus bar assembly covering an opening formed at one side of the module case, wherein the bus bar assembly includes a bus bar frame sized and shaped corresponding to the opening formed at one side of the module case, and a plurality of bus bars fixedly mounted to the bus bar frame, wherein each one of the plurality of bus bars comprises a first bus bar, a second bus bar, and a third bus bar; a pressing jig installed on the bus bar assembly and configured to press a bus bar of the plurality of bus bars at both sides thereof so that electrode leads located in lead slits (S1, S2) provided between the first bus bar and the second bus bar and between the first bus bar and the third bus bar are welded to the bus bar in a closely adhered state, wherein the electrode leads are drawn from the plurality of battery cells; and a welding device configured to perform welding in a state where the electrode leads located in the lead slits are closely adhered to the bus bar, wherein the pressing jig includes: a first frame having a first distance adjusting unit and a first pressing unit; and a second frame having a second distance adjusting unit facing the first distance adjusting unit and a second pressing unit facing the first pressing unit, and the first frame and the second frame are coupled by a hinge so that a distance between first pressing unit and the second pressing unit is increased when a distance between the first distance adjusting unit and the second distance adjusting unit is decreased, and the distance between first pressing unit and the second pressing unit is decreased when the distance between the first distance adjusting unit and the second distance adjusting unit is increased. a battery module including a cell stack formed by stacking a plurality of battery cells, a module case accommodating the cell stack, a bus bar assembly covering an opening formed at one side of the module case, wherein the bus bar assembly includes a bus bar frame sized and shaped corresponding to the opening formed at one side of the module case, and a plurality of bus bars fixedly mounted to the bus bar frame, wherein each one of the plurality of bus bars comprises a first bus bar, a second bus bar, and a third bus bar; a pressing jig installed on the bus bar assembly and configured to press a bus bar of the plurality of bus bars at both sides thereof so that electrode leads located in lead slits (S1, S2) provided between the first bus bar and the second bus bar and between the first bus bar and the third bus bar are welded to the bus bar in a closely adhered state, wherein the electrode leads are drawn from the plurality of battery cells; and a welding device configured to perform welding in a state where the electrode leads located in the lead slits are closely adhered to the bus bar, wherein the pressing jig includes: a first frame having a first distance adjusting unit and a first pressing unit; and a second frame having a second distance adjusting unit facing the first distance adjusting unit and a second pressing unit facing the first pressing unit, and the first frame and the second frame are coupled by a hinge so that a distance between first pressing unit and the second pressing unit is increased when a distance between the first distance adjusting unit and the second distance adjusting unit is decreased, and the distance between first pressing unit and the second pressing unit is decreased when the distance between the first distance adjusting unit and the second distance adjusting unit is increased. a first frame having a first distance adjusting unit and a first pressing unit; and a second frame having a second distance adjusting unit facing the first distance adjusting unit and a second pressing unit facing the first pressing unit, and the first frame and the second frame are coupled by a hinge so that a distance between first pressing unit and the second pressing unit is increased when a distance between the first distance adjusting unit and the second distance adjusting unit is decreased, and the distance between first pressing unit and the second pressing unit is decreased when the distance between the first distance adjusting unit and the second distance adjusting unit is increased.