A method for diagnosing an error of cell balancing in a cell balancing circuit constantly maintaining a plurality of cell voltages of a battery includes a first step of calculating a cell balancing residual time through a cell balancing required time and a cell balancing performing time in a previous driving, a second step of confirming a cell balancing required time in a current driving after the first step, a third step of calculating a time deviation between the cell balancing required time of the second step and the cell balancing residual time of the first step, and a fourth step of deciding whether or not the error has occurred through the time deviation calculated in the third step.