A control circuit performs a read operation of reading data held in a memory-cell by supplying a selected word-line with a read voltage that is a voltage between the lower limit and the upper limit of a plurality of threshold-voltage distributions provided to the memory-cell. The control circuit also performs a verify operation of determining whether a write operation is completed by supplying a selected word-line with a verify voltage higher than the read voltage to read the memory cell. The control circuit then performs a data variation determination operation of determining whether the memory-cells connected to a selected word-line each have a threshold voltage equal to or less than a certain value to determine, from among the plurality of memory cells connected to the selected word-line, whether the number of memory cells where data variation has occurred is not less than a certain number.