A battery module assembly, according to one embodiment of the present invention, comprises: a battery module including a base substrate and at least one battery cell, wherein a plurality of unit battery modules formed to surround a cell cover are stacked on the base substrate; and a battery case which is coupled to the base substrate, and which includes a first case surrounding the front surface of the battery module and a second case combined with the first case and surrounding a rear surface of the battery module. The present invention can effectively provide coupling reliability with an inner sensing membrane which is coupled to the battery module, and can seal the inside of the battery case by tightly combining the first case and the second case in a lateral direction.