A battery system comprising: a plurality of battery units (BTU1, BTUn), the battery unit (BTU1, BTUn) having a battery bank (V1, Vn) made up by connecting a plurality of storage batteries with each other, and a switching circuit (SW1, SWn), connected in series to the battery bank (V1, Vn), the plural battery units (BTU1, BTUn) being connected in parallel with each other, wherein the switching circuit (SW1, SWn) is made up by connecting a first circuit (Ca1, Can) comprising a first switch (Sa1, San) connected in series to a precharge resistor (Zpr1, Zprn) in parallel with a second circuit (Cb1, Cbn) comprising a second switch (Sb1, Sbn), wherein the plural battery units (BTU1, BTUn) each comprise: a battery bank voltage detector (Va1, Van) that detects a voltage difference between the respective ends of the battery bank (V1, Vn); and a first current detector (I1, In) that detects a current flowing through the battery bank (V1, Vn), wherein the controller (CTRA) turns the first switch (Sa1, San) into the ON state in a connection-target battery unit to serve as a target for connection when the connection-target battery unit is connected to the battery system, turning the first switch (Sa1, San) into the OFF state after the second switch (Sb1, Sbn) is turned into the OFF state, whereupon the ON/OFF state of the first switch (Sa1, San) and the second switch (Sb1, Sbn), respectively, is controlled on the basis of a current detection information piece of the first current detector (I1, In), a current detection information piece of the second current detector, and a voltage detection information piece of the battery bank voltage detector (Va1, Van) when the second switch (Sb1, Sbn) is turned into the OFF state, and turns the second switch (Sb1, Sbn) into the ON state when a difference between the current detection information piece of the first current detector of the connection-target battery unit and the current detection information piece of the first current detector (In) of the battery unit (BTUn) that is already connected to the battery system is converged within a predetermined range. a plurality of battery units (BTU1, BTUn), the battery unit (BTU1, BTUn) having a battery bank (V1, Vn) made up by connecting a plurality of storage batteries with each other, and a switching circuit (SW1, SWn), connected in series to the battery bank (V1, Vn), the plural battery units (BTU1, BTUn) being connected in parallel with each other, wherein the switching circuit (SW1, SWn) is made up by connecting a first circuit (Ca1, Can) comprising a first switch (Sa1, San) connected in series to a precharge resistor (Zpr1, Zprn) in parallel with a second circuit (Cb1, Cbn) comprising a second switch (Sb1, Sbn), wherein the plural battery units (BTU1, BTUn) each comprise: a battery bank voltage detector (Va1, Van) that detects a voltage difference between the respective ends of the battery bank (V1, Vn); and a first current detector (I1, In) that detects a current flowing through the battery bank (V1, Vn), wherein the controller (CTRA) turns the first switch (Sa1, San) into the ON state in a connection-target battery unit to serve as a target for connection when the connection-target battery unit is connected to the battery system, turning the first switch (Sa1, San) into the OFF state after the second switch (Sb1, Sbn) is turned into the OFF state, whereupon the ON/OFF state of the first switch (Sa1, San) and the second switch (Sb1, Sbn), respectively, is controlled on the basis of a current detection information piece of the first current detector (I1, In), a current detection information piece of the second current detector, and a voltage detection information piece of the battery bank voltage detector (Va1, Van) when the second switch (Sb1, Sbn) is turned into the OFF state, and turns the second switch (Sb1, Sbn) into the ON state when a difference between the current detection information piece of the first current detector of the connection-target battery unit and the current detection information piece of the first current detector (In) of the battery unit (BTUn) that is already connected to the battery system is converged within a predetermined range. a battery bank voltage detector (Va1, Van) that detects a voltage difference between the respective ends of the battery bank (V1, Vn); and a first current detector (I1, In) that detects a current flowing through the battery bank (V1, Vn), wherein the controller (CTRA) turns the first switch (Sa1, San) into the ON state in a connection-target battery unit to serve as a target for connection when the connection-target battery unit is connected to the battery system, turning the first switch (Sa1, San) into the OFF state after the second switch (Sb1, Sbn) is turned into the OFF state, whereupon the ON/OFF state of the first switch (Sa1, San) and the second switch (Sb1, Sbn), respectively, is controlled on the basis of a current detection information piece of the first current detector (I1, In), a current detection information piece of the second current detector, and a voltage detection information piece of the battery bank voltage detector (Va1, Van) when the second switch (Sb1, Sbn) is turned into the OFF state, and turns the second switch (Sb1, Sbn) into the ON state when a difference between the current detection information piece of the first current detector of the connection-target battery unit and the current detection information piece of the first current detector (In) of the battery unit (BTUn) that is already connected to the battery system is converged within a predetermined range.