1. A method of operating a nonvolatile memory device which sequentially performs a plurality of erase loops to erase a memory block, the method comprising: performing at least one of the erase loops; performing a post-program operation on the memory block after the at least one erase loop is performed; and performing the remaining erase loops by: detecting a threshold voltage distribution of the memory block; adjusting an erase voltage increment based on the detected threshold voltage distribution; and performing at least one of the remaining erase loops using the adjusted erase voltage increment, wherein the post-program operation is not performed when each of the remaining erase loops is performed, and wherein detecting the threshold voltage distribution comprises detecting off cells that have a threshold voltage that is higher than an off cell voltage from among memory cells that are connected to an erase reference word line, wherein the erase reference word line is one of a plurality of word lines that are connected to the memory block.