1. A non-volatile NAND-flash semiconductor memory device which is configured to execute at least one of a pre-program operation and a post-program operation as part of an erase operation, each of the pre-program and post-program operations of the erase operation including applying a program voltage to a subset only of a plurality of word lines defining a word line block of the memory device before and after erasing memory cells of the word line block, respectively, wherein the at least one of the pre-program operation and the post-program operations is executed with respect to an upper word line, wherein the upper word line includes at least one word line between a string selection line and a selected word line, wherein the selected word line is applied with a program voltage during a previous program operation.