To provide a composition for preventing or improving abnormality in a bacterial strain composition of intestinal bacterial flora.SOLUTION: The present invention provides a composition for preventing or improving abnormality in a bacterial strain composition of intestinal bacterial flora in a subject, containing hydrogen gas or dissolved hydrogen as an active ingredient.SELECTED DRAWING: Figure 3【課題】腸内細菌叢の細菌種組成異常を予防又は改善するための組成物を提供することを課題とする。【解決手段】水素ガス又は溶存水素を有効成分として含む、被験体内で腸内細菌叢の細菌種組成異常を予防又は改善するための組成物。【選択図】図3