A method of manufacturing a battery is provided including the step of locating a plurality of unit cells in a support body having an opening portion. A plurality of jigs each having a convex portion and a bore are mounted at the support body so as to cause the convex portion to be fit into the opening portion of the support body. A spacing between the support body and the plurality of the unit cells is filled with a plastic resin by charging the plastic resin into the spacing between the support body and the plurality of unit cells through the bore of one jig of the plurality of jigs, while releasing air inside the support body through the bore of another jig of the plurality of jigs that is opposite to the one jig.