In a memory cell array, a plurality of memory cells are arranged in a matrix. Each of the plurality of memory cells stores one of a plurality of threshold levels. When writing one of the plurality of threshold levels into a first memory cell of the memory cell array, a control circuit writes a threshold level a little lower than the original threshold level. When not writing a second memory cell adjacent to the first memory cell consecutively, the control circuit writes the original threshold level into the first memory cell.