Oxygen starting material, which already contains heavy isotopes of oxygen, is enriched in the heavy isotopes of oxygen by means of cryogenic distillation in an oxygen distillation column system 1A, then water containing a high concentration of the above-mentioned heavy isotopes of oxygen is formed by adding hydrogen to the enriched product and reacting them in a reactor system 3A. This formed water is distilled in the water distillation column system 4A and, thereby, heavy oxygen water enriched in the above-mentioned heavy isotopes of oxygen is produced.