A battery system monitoring apparatus for monitoring a cell group having a plurality of battery cells connected in series, the battery system monitoring apparatus comprising: a cell controller IC which is configured to monitor and control the states of the plurality of battery cells of the cell group; a battery controller which is configured to control the cell controller IC; a plurality of voltage detection lines for measuring the voltage across the terminals of the battery cells, the voltage detection lines connecting positive and negative electrodes of the battery cells, respectively, to a plurality of voltage input terminals of the cell controller integrated circuit, IC; a power line which connects the positive electrode of a battery cell having the highest potential among the plurality of battery cells to a power supply terminal of the cell controller IC; and a ground line which connects the negative electrode of the battery cell having the lowest potential among the plurality of battery cells to a ground terminal of the cell controller IC, wherein the battery cells are each provided with a balancing switch which is configured to perform balancing discharge on the battery cell, the balancing switch being connected between the voltage detection line connected to the positive electrode of the battery cell and the voltage detection line connected to the negative electrode, and a balancing resistor connected in series to the balancing switch, the voltage detection lines are provided with voltage input resistors in series, and the battery controller is configured to control the cell controller IC, wherein the balancing switch is controlled to set an effective balancing resistance value of the balancing switch, wherein the effective balancing resistance value is set to a predetermined value or more for cells whose voltage detection lines are involved in the occurrence of leakage, wherein the effective balancing resistance value (Rb ef) is determined from the resistance value (Rcv) of the voltage input resistor provided to the voltage detection line having the leakage multiplied by a voltage value when the battery cell has a SOC of 100% (V F) and divided by the difference between an overcharge protection voltage value of the battery cell (V max) and the voltage value when the battery cell has the SOC of 100% F, and wherein even upon the occurrence of leakage between two voltage detection lines out of the plurality of voltage detection lines, between the power line and any of the plurality of voltage detection lines, or between the ground line and any of the plurality of voltage detection lines the battery cell is not overcharged. a cell controller IC which is configured to monitor and control the states of the plurality of battery cells of the cell group; a battery controller which is configured to control the cell controller IC; a plurality of voltage detection lines for measuring the voltage across the terminals of the battery cells, the voltage detection lines connecting positive and negative electrodes of the battery cells, respectively, to a plurality of voltage input terminals of the cell controller integrated circuit, IC; a power line which connects the positive electrode of a battery cell having the highest potential among the plurality of battery cells to a power supply terminal of the cell controller IC; and a ground line which connects the negative electrode of the battery cell having the lowest potential among the plurality of battery cells to a ground terminal of the cell controller IC, wherein the battery cells are each provided with a balancing switch which is configured to perform balancing discharge on the battery cell, the balancing switch being connected between the voltage detection line connected to the positive electrode of the battery cell and the voltage detection line connected to the negative electrode, and a balancing resistor connected in series to the balancing switch, the voltage detection lines are provided with voltage input resistors in series, and the battery controller is configured to control the cell controller IC, wherein the balancing switch is controlled to set an effective balancing resistance value of the balancing switch, wherein the effective balancing resistance value is set to a predetermined value or more for cells whose voltage detection lines are involved in the occurrence of leakage, wherein the effective balancing resistance value (Rb ef) is determined from the resistance value (Rcv) of the voltage input resistor provided to the voltage detection line having the leakage multiplied by a voltage value when the battery cell has a SOC of 100% (V F) and divided by the difference between an overcharge protection voltage value of the battery cell (V max) and the voltage value when the battery cell has the SOC of 100% F, and wherein even upon the occurrence of leakage between two voltage detection lines out of the plurality of voltage detection lines, between the power line and any of the plurality of voltage detection lines, or between the ground line and any of the plurality of voltage detection lines the battery cell is not overcharged.