The present invention provides an agent for promoting graft survival which can suppress rejection without use of existing immunosuppressants, an organ preservation solution capable of maintaining the freshness of an organ excised from a donor, and the like. An agent for promoting graft survival or an organ preservation solution is prepared, which comprises 5-aminolevulinic acid (ALA) or a derivative thereof, or a salt of ALA or the derivative and an iron compound as active ingredients. Preferable examples of the ALAs can include ALA and various esters such as methyl ester, ethyl ester, propyl ester, butyl ester, and pentyl ester of ALA, and their hydrochlorides, phosphates, and sulfates. Preferable examples of the iron compound can include sodium ferrous citrate.