A method of operating a nonvolatile memory device including a memory cell array having first and second main cells for storing external input data, first spare cells for storing data for error correction code (ECC) processing on the data stored in the first and second main cells and second spare cells for storing data for ECC processing on the data stored in the first and second main cells which involves reading the data stored in the first spare cells, reading the data stored in the second main cells and the data stored in the second spare cells, and performing the ECC processing on the data read from the second main cells using the data read from the first spare cells and the data read from the second spare cells.