Measurement of a cell voltage is executed immediately after diagnosis of a battery management device is ended. In a battery management device, current sources repeatedly perform an energization operation to cause a current to flow to voltage detection lines with a magnitude of the current that enables each amount of charge stored in capacitors changed by one energization operation to fall within a range corresponding to a fluctuation width of terminal voltages of battery cells during the energization operation when resistors are in a normal state. When the difference between the current terminal voltage of the battery cell and the past terminal voltage of the battery cell is larger than the predetermined threshold value, the microcomputer diagnoses that the resistor is in the open state.