Provided are an apparatus and a method for diagnosing an abnormality in a cell balancing circuit. The apparatus may include a floating capacitor charged with voltage of a battery cell, a cell balancing circuit for discharging the floating capacitor, a voltage measuring unit for measuring the battery cell voltage of the charged floating capacitor and a residual voltage of the discharged floating capacitor, and a control unit for determining an abnormality in the cell balancing circuit based on the residual voltage of the discharged floating capacitor.