A memory system includes a memory device, a memory controller configured to control the memory device, and an auxiliary power source configured to supply power to the memory device and the memory controller. The memory controller activates the auxiliary power source in response to the occurrence an NPO (normal power-off) or an SPO (sudden power-off), checks whether there exists an uncompleted operation at a point of time at which the auxiliary power source is activated, and completes the uncompleted operation, and when an amount of residual energy of the auxiliary power source after completing the uncompleted operation exceeds a predetermined threshold value, performs a data verify operation for a predetermined area in the memory device and stores a result of the data verify operation in the memory device.