An object of the present invention is to provide a regulatory dendritic cell inducing agent applicable to the treatment of immune disease as well as an immune tolerance inducing agent such as a preventive and/or therapeutic agent for allergic disease and a preventive and/or therapeutic agent for autoimmune disease, both of which are safe and have the mechanism of action different from that of conventional drugs. As a means for achieving the above object, an immune tolerance inducing agent comprising 5-aminolevulinic acid (ALA) or a derivative thereof, or a salt of the 5-ALA or the derivative and an iron compound as active ingredients is prepared. 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.
