A cell balance device for a battery system for adjusting a cell balance of a plurality of electric accumulators connected in series, the cell balance device comprising: first, second and third electric accumulator connection terminals (T11, T21, T31) each configured to be connected either to a terminal of only one of the electric accumulators or to interconnected terminals of two of the electric accumulators (A1, A2) connected in series; a voltage hold device connection terminal to be connected to a voltage hold device; a first switch circuit provided between the first electric accumulator connection terminal and the voltage hold device connection terminal; a second switch circuit provided between the second electric accumulator connection terminal and the voltage hold device connection terminal; a third switch circuit provided between the third electric accumulator connection terminal and the voltage hold device connection terminal; a receiving terminal configured to receive a synchronization signal; a transmitting terminal configured to transmit the synchronization signal; a control circuit configured to control ON/OFF of the first, second and third switch circuits based on the synchronization signal; and overcurrent detection circuits (211, 221, 231) provided between the first, second and third electric accumulator connection terminals (T11, T21, T31) and the voltage hold device connection terminal; each overcurrent detection circuit (211, 221, 231) being configured to detect an overcurrent flowing through a respective one of each of the first, second and third switch circuits (S11, S21, S31) and comprising: a further switch circuit which is controlled so as to be turned ON/OFF simultaneously with the respective one of the first, second and third switch circuits; a constant current circuit configured to allow a constant current to flow through the further switch circuit; and a comparator circuit configured to compare a first voltage and a second voltage, the first voltage being based on an ON-state resistance of the respective one of the first, second and third switch circuits (S11, S21, S31) and a current flowing through the respective one of the first, second and third switch circuits (S11, S21, S31), and the second voltage being based on an ON-state resistance of the further switch circuit and the constant current. first, second and third electric accumulator connection terminals (T11, T21, T31) each configured to be connected either to a terminal of only one of the electric accumulators or to interconnected terminals of two of the electric accumulators (A1, A2) connected in series; a voltage hold device connection terminal to be connected to a voltage hold device; a first switch circuit provided between the first electric accumulator connection terminal and the voltage hold device connection terminal; a second switch circuit provided between the second electric accumulator connection terminal and the voltage hold device connection terminal; a third switch circuit provided between the third electric accumulator connection terminal and the voltage hold device connection terminal; a receiving terminal configured to receive a synchronization signal; a transmitting terminal configured to transmit the synchronization signal; a control circuit configured to control ON/OFF of the first, second and third switch circuits based on the synchronization signal; and overcurrent detection circuits (211, 221, 231) provided between the first, second and third electric accumulator connection terminals (T11, T21, T31) and the voltage hold device connection terminal; each overcurrent detection circuit (211, 221, 231) being configured to detect an overcurrent flowing through a respective one of each of the first, second and third switch circuits (S11, S21, S31) and comprising: a further switch circuit which is controlled so as to be turned ON/OFF simultaneously with the respective one of the first, second and third switch circuits; a constant current circuit configured to allow a constant current to flow through the further switch circuit; and a comparator circuit configured to compare a first voltage and a second voltage, the first voltage being based on an ON-state resistance of the respective one of the first, second and third switch circuits (S11, S21, S31) and a current flowing through the respective one of the first, second and third switch circuits (S11, S21, S31), and the second voltage being based on an ON-state resistance of the further switch circuit and the constant current. a further switch circuit which is controlled so as to be turned ON/OFF simultaneously with the respective one of the first, second and third switch circuits; a constant current circuit configured to allow a constant current to flow through the further switch circuit; and a comparator circuit configured to compare a first voltage and a second voltage, the first voltage being based on an ON-state resistance of the respective one of the first, second and third switch circuits (S11, S21, S31) and a current flowing through the respective one of the first, second and third switch circuits (S11, S21, S31), and the second voltage being based on an ON-state resistance of the further switch circuit and the constant current.