A controller includes a generation unit configured to aggregate comparison results between second threshold voltage levels held in the memory cells and predetermined third threshold voltage levels, and generate a histogram of the second threshold voltage levels, an estimation unit configured to estimate statistical parameter of a distribution of the second threshold voltage levels with respect to a first threshold voltage level according to writing data, based on the histogram, and a determination unit configured to determine a fifth threshold voltage level defining a boundary of a fourth threshold voltage level indicating a read result of the memory cells from the third threshold voltage levels based on the statistical parameter in such a manner that mutual information amount between the first threshold voltage level and the fourth threshold voltage level becomes maximum.