A battery state detector includes a main unit configured to detect a state of one battery assembly in a battery pack storing therein a plurality of battery assemblies each including a plurality of battery cells arranged in rows, a subunit configured to detect a state of a battery assembly in the battery pack other than the battery assembly that is a detection target of the main unit, and a first communication line that connects the main unit with the subunit. The main unit is connected to a controller configured to control the battery pack via a second communication line that is separate from the first communication line.