Disclosed is a battery pack assembly including an apparatus for detecting water accumulation inside an installation region in which a battery pack is installed. The battery pack assembly includes a watertight case installed on a bottom surface of the water accumulation detecting apparatus and assembled with the battery pack therein. The water accumulation detecting apparatus includes a body including a water inlet opening through which water accumulated in the installation region flows in. A floating member floated by buoyancy of the water flowing in the body. A switch unit is configured to generate a current when the floating member increases up to or beyond a predetermined height. An elastic member is configured to connect the floating member and the bottom surface of the body so that the floating member is positioned at a predetermined position when buoyancy of the water does not affect to the floating member.