A method of stabilizing a hem protein which is effective against the denaturation and degradation of a hem protein typified by hemoglobin and a storage solution therefor. A method of stabilizing a hem protein and a storage solution therefor characterized in that an iminocarboxylic acid or its salt is made to coexist in a sample containing the hem protein, wherein the above-described iminocarboxylic acid is a compound represented by the following general formula (1) wherein R represents a hydrogen atom or a hydroxyl group; and X's represent each a hydrogen atom, an alkali metal or an ammonium group.