A nonvolatile semiconductor storage device includes a memory cell array and a peripheral circuit. The memory cell array includes active areas extending in a first direction, a dummy active area extending in the first direction, memory cells on the plurality of active areas, first dummy cells on the dummy active area, diffusion layer areas each connected to the corresponding memory cell and the corresponding first dummy cell, first contacts in the respective active areas, and a second contact in the dummy active area. The peripheral circuit includes a voltage applying unit configured to apply to each of the first contacts a first voltage to set each of the memory cells in a write enable state or a second voltage to set the memory cells in a write inhibit state, and to apply to the second contact a third voltage to change a threshold of the dummy cell.