Provided is a solid composition comprising an iminocarboxylic acid salt, wherein the iminocarboxylic acid salt is 3-hydroxy-2,2′-iminodisuccinic acid and/or a salt thereof. The solid composition comprises 70% by weight or more of the iminocarboxylic acid salt, relative to a solid content except for moisture of the solid composition, and the solid composition comprises 20% by weight or less of moisture, relative to the solid composition. Also a production method of the solid composition, comprising a step of drying an iminocarboxylic acid salt-containing composition is provided.