An operating method of a storage device, which includes a nonvolatile memory device, includes entering a power-on mode, searching for an open memory block, which includes at least one erase word line, from among memory blocks included in the nonvolatile memory device, applying a program voltage to the at least one erase word line to close the open memory block if the number of the erase word lines included in the open memory block is not more than a preset value, and after the power-on mode, entering a normal operation mode. Memory cells connected to the at least one erase word line to which the program voltage is applied are programmed to have a threshold voltage distribution range higher than a threshold voltage distribution range of an erase state.