FLASH MEMORY

A flash memory refreshes at a time before a read error might occur. A controller performs a first read operation and a second read operation using a sense amplifier. In the second read operation, a bit line potential controller draws out a potential of a bit line feeding the sense amplifier so that, if memory cell degradation has occurred, the degradation can be detected. For example, when first data read by the first read operation and second data read by the second read operation are determined to be different, the memory cell is refreshed.