The present invention addresses the problem of providing an agent for inducing regulatory dendritic cells that can be applied to treating immune diseases, is safe, and has a different mechanism of action than existing drugs, and also providing an immune tolerance inducer such as an agent for preventing/treating allergic disease or an agent for preventing/treating autoimmune disease. The problem is solved by preparing an immune tolerance inducer in which the active ingredients are 5-aminolevulinic acid (ALA), or a derivative or salt thereof, and an iron compound. The ALA can be preferably exemplified by ALA or a variety of esters thereof, such as the methyl, ethyl, propyl, butyl, or pentyl ester, as well as the hydrochlorides, phosphates, sulfates, or the like thereof, and the iron compound can be preferably exemplified by sodium ferrous citrate.
