A device for controlling fuel cell stack connection, comprising a plurality of fuel cell stacks including an n number of fuel cell stacks (210, 220, 230, 240), and a power conditioning unit connected to the plurality of fuel cell stacks, characterized in that the device comprising: a memory storing therein a first critical value by which the fuel cell stack connection control device starts an operation, second critical values ranged from zero to the first critical value, and a boundary value used to place the second critical values in at least two subsidiary ranges, wherein the first critical value is a value equal to or less than a voltage difference between a 1 st fuel cell stack having a highest voltage and an n th fuel cell stack having a lowest voltage of the fuel cell stacks; a plurality of stack voltage sensing units (310, 320, 330, 340) for sensing voltages of the plurality of fuel cell stacks; and a control unit for determining whether a difference between two sensed voltages has been increased to the first critical value or greater, and starting to control an operation of a switching unit when a difference between two sensed voltages has been increased to the first critical value or greater, thereby placing one or more fuel cell stacks having voltages associated with each one of the at least two subsidiary ranges in one of individual groups, wherein when the control unit determines that the voltage difference has been increased to the first critical value or greater, the control unit determines a highest voltage value of the fuel cell stacks, and determines difference values by subtracting subtrahend values that are the voltage values of the 1 st to n th fuel cell stacks from a minuend value that is the determined highest voltage value, and the control unit controls the operation of the switching unit in such a way that one or more fuel cell stacks having subtrahend values that determine the difference values associated with each one of the at least two subsidiary ranges are placed in one of the individual groups, and the individual groups are connected to respective DC-DC converters (510, 520) installed in the power conditioning unit. a memory storing therein a first critical value by which the fuel cell stack connection control device starts an operation, second critical values ranged from zero to the first critical value, and a boundary value used to place the second critical values in at least two subsidiary ranges, wherein the first critical value is a value equal to or less than a voltage difference between a 1 st fuel cell stack having a highest voltage and an n th fuel cell stack having a lowest voltage of the fuel cell stacks; a plurality of stack voltage sensing units (310, 320, 330, 340) for sensing voltages of the plurality of fuel cell stacks; and a control unit for determining whether a difference between two sensed voltages has been increased to the first critical value or greater, and starting to control an operation of a switching unit when a difference between two sensed voltages has been increased to the first critical value or greater, thereby placing one or more fuel cell stacks having voltages associated with each one of the at least two subsidiary ranges in one of individual groups, wherein when the control unit determines that the voltage difference has been increased to the first critical value or greater, the control unit determines a highest voltage value of the fuel cell stacks, and determines difference values by subtracting subtrahend values that are the voltage values of the 1 st to n th fuel cell stacks from a minuend value that is the determined highest voltage value, and the control unit controls the operation of the switching unit in such a way that one or more fuel cell stacks having subtrahend values that determine the difference values associated with each one of the at least two subsidiary ranges are placed in one of the individual groups, and the individual groups are connected to respective DC-DC converters (510, 520) installed in the power conditioning unit.