A bus bar assembly for electrically connecting a plurality of battery cells having electrode leads, the bus bar assembly comprising: an insertion-type bus bar provided in the form of a rod-shaped conductor; side-adhering bus bars positioned with the insertion-type bus bar being interposed therebetween to form fitting spaces respectively beside both sides of the insertion-type bus bar so that at least one electrode lead can be inserted in the fitting spaces; and a supporting and holding member configured to support the insertion-type bus bar at a location protruding further to the side-adhering bus bars, the supporting and holding member being elastically deformed by the insertion-type bus bar so that the insertion-type bus bar is positioned on the same plane as the side-adhering bus bars when the insertion-type bus bar is moved in the (-) direction along a Z axis with respect to the side- adhering bus bars, wherein the supporting and holding member is a leaf spring that has both ends supporting the insertion-type bus bar and is provided to be elastically widened to the left and right directions, and the leaf spring compresses both side surfaces of the insertion-type bus bar with an elastic restoring force when the insertion-type bus bar is inserted into the leaf spring, wherein the insertion-type bus bar includes: a first flat portion provided to face side surfaces of the side-adhering bus bars on the same plane as the side-adhering bus bar; and a second flat portion formed to have a smaller width than the first flat portion and compressed by both ends of the leaf spring. an insertion-type bus bar provided in the form of a rod-shaped conductor; side-adhering bus bars positioned with the insertion-type bus bar being interposed therebetween to form fitting spaces respectively beside both sides of the insertion-type bus bar so that at least one electrode lead can be inserted in the fitting spaces; and a supporting and holding member configured to support the insertion-type bus bar at a location protruding further to the side-adhering bus bars, the supporting and holding member being elastically deformed by the insertion-type bus bar so that the insertion-type bus bar is positioned on the same plane as the side-adhering bus bars when the insertion-type bus bar is moved in the (-) direction along a Z axis with respect to the side- adhering bus bars, wherein the supporting and holding member is a leaf spring that has both ends supporting the insertion-type bus bar and is provided to be elastically widened to the left and right directions, and the leaf spring compresses both side surfaces of the insertion-type bus bar with an elastic restoring force when the insertion-type bus bar is inserted into the leaf spring, wherein the insertion-type bus bar includes: a first flat portion provided to face side surfaces of the side-adhering bus bars on the same plane as the side-adhering bus bar; and a second flat portion formed to have a smaller width than the first flat portion and compressed by both ends of the leaf spring. a first flat portion provided to face side surfaces of the side-adhering bus bars on the same plane as the side-adhering bus bar; and a second flat portion formed to have a smaller width than the first flat portion and compressed by both ends of the leaf spring.