Provided are an apparatus and a method for diagnosing an abnormality in a cell balancing circuit. The apparatus may include a plurality of cell balancing circuits respectively connected to a plurality of battery cells included in a battery pack for balancing the voltages of the battery cells, a diagnosis resistor respectively installed between the cell balancing circuit and a cathode terminal and an anode terminal of the corresponding battery cell, a voltage measuring unit for measuring a voltage difference of the balancing circuit corresponding to each of the battery cells, and a control unit for turning on or off a cell balancing circuit to be diagnosed and for determining whether there is an abnormality in the cell balancing circuit to be diagnosed, based on a variation pattern of a voltage difference of a cell balancing circuit adjacent to the cell balancing circuit to be diagnosed that is measured by the voltage measuring unit.