To perform electric power storage by liquefying hydrogen and oxygen by a water electrolytic device.

The water electrolytic hydrogen-oxygen energy storage system comprises: a water electrolytic device 11; an oxygen liquefaction device 12a, a hydrogen liquefaction device 12b and a liquefied gas storage device 13. The water electrolytic device 11 electrolyzes water using received electric power to generate a high pressure hydrogen gas and a high pressure oxygen gas. The oxygen liquefaction device 12a expands and decompresses the oxygen gas and cools the same to generate liquid oxygen. The hydrogen liquefaction device 12b cools the hydrogen gas using the liquid oxygen, and also expands and decompresses the same to generate liquid hydrogen. The liquefied gas storage device 13 stores the liquid oxygen and the liquid hydrogen.