A battery monitoring apparatus is used in a battery assembly equipped with a plurality of batteries connected in series with each other. A plurality of measuring lines are mounted on a given layer of a multi-layer substrate and work to measure electrical potentials developed at terminals of the batteries, respectively. The measuring lines are classified into measuring line groups in order of electrical potential from lowest to highest. A plurality of solid patterns are arranged on one of layers of the multi-layer substrate other than the given layer and provided one for each of the measuring line groups. The solid patterns overlap the measuring line groups in planar view in a stacking direction of the multi-layer substrate. The solid patterns are arranged away from each other. The higher electrical potentials of the measuring line groups are, the higher electrical potentials of the corresponding solid patterns are.