Provided are: an enhancer of the survival of a transplanted organ, which can prevent the occurrence of any rejection reaction without requiring the use of any existing immunosuppressing agent an organ preservation solution which enables the preservation of the freshness of an organ after the excision of the organ from a donor and others. Prepared are: an enhancer of the survival of a transplanted organ, which contains 5-aminolevulinic acid (ALA) or a derivative or salt thereof and an iron compound as active ingredients and an organ preservation solution. Preferred examples of the ALA compound include ALA, esters thereof including a methyl ester, an ethyl ester, a propyl ester, a butyl ester, a pentyl ester and the like thereof, and hydrochloric acid salts, phosphoric acid salts and sulfuric acid salts of ALA and the esters. A preferred example of the iron compound is sodium ferrous citrate.