An object of the present invention is to provide a preventive and/or therapeutic agent for radiation damage that has a novel mechanism of action and can prevent and alleviate a wide range of symptoms of radiation damage for which no therapeutic measures have been available so far, and moreover, is highly safe to humans. A preventive and/or therapeutic agent for radiation damage comprising a compound represented by the following formula (I) such as 5-aminolevulinic acid (5-ALA) or a salt thereof as an active ingredient can be used to improve the survival rate, improve body weight reduction, and alleviate hematopoietic disorder:(wherein R1 represents a hydrogen atom or an acyl group； and R2 represents a hydrogen atom, a linear or branched alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group).
