1. A data storing method comprising: preparing a non-volatile memory device that includes a memory cell array including a plurality of memory cells, wherein the plurality of memory cells include a memory cell in an initial state, which does not change, unless a forming stress is applied thereto, to a variable state, in which a resistance value reversibly changes between a plurality of changeable resistance value ranges in accordance with an electric signal applied thereto; and applying the forming stress to the memory cell in the initial state, to store data in the memory cell array on the basis of whether each of the plurality of memory cells is in the initial state or the variable state.