1. A data storing method by a non-volatile memory device including a memory cell array having a plurality of memory cells, wherein data is defined based on whether a resistance value of each of the memory cells is in an initial state or a variable state, the initial state corresponding to an initial resistance value range, a resistance value of a memory cell among the plurality of memory cells in the variable state falls within one of plural resistance value ranges including at least a first resistance value range and a second resistance value range, the resistance value of the memory cell in the variable state changes from the resistance value within the second resistance value range to the resistance value within the first resistance value range in response to application of a first signal, the resistance value of the memory cell in the variable state changes from the resistance value within the first resistance value range to the resistance value within the second resistance value range in response to application of a second signal, the data storing method comprises: applying an electric stress by an electric pulse to the memory cells in the initial state; stopping the application of the electric stress when a portion of the memory cells change from the initial state to the variable state; and storing random data, as the data, unique to the non-volatile memory device, the memory cells in the initial state do not change to the variable state, unless the electric stress higher than a threshold stress is applied thereto, and the memory cell array having the plurality of memory cells includes one or more memory cells maintained in the initial state and one or more memory cells which have changed from the initial state to the variable state, a combination of the one or more memory cells maintained in the initial state and the one or more memory cells which have changed from the initial state to the variable state changes randomly by adjusting a time period for which the electric stress is applied, the combination indicates the data, the electric stress includes the first signal, the second signal, and an auxiliary stress which is smaller in magnitude than the second signal, and the time period for which the electric stress is applied is adjusted by adjusting a time period for which the first signal is applied, a time period for which the second signal is applied, and a time period for which the auxiliary stress is applied.