An object of the present invention is to provide a scavenger of in vivo harmful reactive oxygen species and/or free radicals, which is capable of effectively reducing the concentrations of in vivo reactive oxygen species and/or free radicals and exhibiting given effects such as the suppression of aging process, the prevention of geriatric or lifestyle-related disease, health promotion, and the inhibition of oxidative stress by virtue of this reduction in the concentrations of reactive oxygen species and/or free radicals. The scavenger of in vivo harmful reactive oxygen species and/or free radicals of the present invention comprises a liquid or gas comprising at least a hydrogen molecule. This medium may further comprise an oxygen molecule. Furthermore, this medium may comprise water or an aqueous solution or may be a gas. The scavenger of reactive oxygen species and/or free radicals can be used in the treatment or prevention of a disorder attributed to reactive oxygen species and/or free radicals.