A nonvolatile semiconductor storage device includes a word line, a first electrode, a high resistance ion diffusion layer, a second electrode, and a bit line. The word line is made of a conductive material extending in a first direction. The first electrode is provided on the word line. The high resistance ion diffusion layer is provided on the first electrode. The second electrode is provided on the ion diffusion layer and configured to supply a metal into the ion diffusion layer upon application of a positive voltage relative to the first electrode. The bit line is provided on the second electrode and made of a conductive material extending in a second direction orthogonal to the first direction. The ion diffusion layer contains oxygen at a higher concentration on the word line side than on the bit line side.