The battery system has a battery 1 having a plurality of series-connected battery cells 2, a voltage detection circuit 3 that detects each battery cell voltage, discharge circuits 4 to discharge each battery cell, and a decision circuit 5 that judges the condition of the connection between a battery cell 2 and the voltage detection circuit 3 from the detected battery cell voltage measured by the voltage detection circuit. The voltage detection circuit 3 measures discharge voltage of a battery cell 2 with the discharge circuit 4 in the discharging state, and measures non-discharge voltage with the battery cell 2 in a non-discharging state. The decision circuit 5 compares the difference between the detected battery cell non-discharge voltage and discharge voltage with the normal voltage, or compares battery cell discharge voltage with the normal voltage to judge abnormal connection between the battery cell and the voltage detection circuit.