Provided is a novel radioprotectant that comprises a compound represented by formula (I) or a pharmaceutically acceptable salt thereof as an active ingredient [in formula (I) : a ring A represents a fused ring; and R1 represents a hydrogen atom or an alkyl group having 1-6 carbon atoms]. In particular, provided is a radioprotectant that comprises a compound represented by formula (A) or (B) as an active ingredient. The radioprotectant according to the present invention reduces cell damage and cell death caused by radiation exposure and exhibits an effect of protecting immune-related cells.