In a nonvolatile memory, the threshold is restored to a state before changing, without increasing number of writing undesirably. In a system including a nonvolatile memory, a random number generator, and a controller accessible to the nonvolatile memory, every time access to the nonvolatile memory is performed, the controller determines a refresh-targeted area, based on a random number generated by the random number generator. The controller is made to perform refresh control to re-write to the refresh-targeted area. By such refresh control, the threshold is restored to a state before changing, without increasing the number of writing undesirably.