A battery device includes secondary battery cells, a detector configured to detect respective voltages of the cells, a first circuit configured to output an overvoltage alarm signal for notifying whether each of the voltage values of the cells is an overvoltage equal to or more than a threshold. value, a second circuit configured to output a first level shutdown signal to an upper controller when any of the secondary battery cells is judged to be overvoltage by the overvoltage alarm signal, and a third circuit configured to be supplied with the shutdown signal and to control operations of the first circuit and the second circuit. The second circuit assumes the shutdown signal at a second level when making self-diagnosis on the first circuit, and outputs an alarm signal for notifying the third circuit of, when the overvoltage alarm signal noticing an overvoltage is supplied, the overvoltage.