According to one embodiment, a memory system includes: a memory device to store data; and a controller to control an operation for the memory device. The memory device executes a program operation by a first program voltage on memory cells belonging to a first address of the memory device; detect at least one first memory cell among the memory cells by using a first determination level and a second determination level different from the first determination level, the at least one first memory cell having a threshold voltage of a value different from a value between the first determination level and the second determination level; and generate unique information of the memory device, based on a position of the first memory cell in the first address.