This application provides a composition for suppressing or preventing metastasis of a cancer, or for suppressing growth of an end stage cancer with metastasis and/or a metastatic cancer(s) therefrom, in a subject having a cancer, the composition comprising gaseous hydrogen as an active ingredient, as well as a method for suppressing or preventing metastasis or suppressing growth of an end stage cancer with metastasis and/or a metastatic cancer(s) therefrom in a subject, comprising administering this composition to the subject having a cancer.