This application is intended to treat or alleviate radiation damage in human patients who have received radiation therapy or have undergone or undergo radiation therapy in a high pressure capsule above standard atmospheric pressure, and at a concentration of 18.5% by volume or less Disclosed is a radiation damage protective agent comprising hydrogen gas as an active ingredient, and a high-pressure capsule for administering a hydrogen gas therapeutic agent such as a radiation damage protection agent to a patient including a human.この出願は、標準大気圧を超える高気圧カプセル内で、放射線被曝を受けた又は放射線療法を受けたもしくは受けるヒト患者の放射線障害を治療又は軽減するための、且つ１８．５体積％以下の濃度で含む水素ガスを有効成分として含むことを特徴とする放射線障害防御剤、並びに、放射線障害防御剤等の水素ガス治療剤をヒトを含む患者に投与するための高気圧カプセルを提供する。
