With a local self boost (LSB) technique, the distribution of threshold voltages after data erase is set toward a higher side and the distribution width is narrowed sufficiently within the range in which cell erase states can be read. To this end, block write is carried out on a memory cell array. Next, setting a predetermined voltage as a start voltage, soft erase is carried out for each block. After carrying out erase verification read, the threshold voltages of the cells are compared with a determination reference value. As a result of this comparison, if the threshold voltages of the cells do not reach the determination reference value, soft erase is repeated. In that case, the predetermined voltage during the soft erase is changed from the start voltage. When the threshold voltages of all the cells have reached the determination reference value, the soft erase is ended.