A circuit is operated to detect unstable memory cells from among a plurality of memory cells in at least one page. A determination is made from an initial status of data stored in a memory cell whether no read error occurs when the data is read at a standard read voltage level, whether a read error occurs and the read error is correctable, and whether a read error occurs and the read error is uncorrectable. Responsive to determining that a read error occurs that is correctable, a further determination is made as to whether the memory cell is correctable by reading the data stored in the memory cell at a correction read voltage level, which has a different voltage level from the standard read voltage level, and by determining whether a read error occurring in the data read at the correction read voltage level is correctable or uncorrectable.