PROBLEM TO BE SOLVED: To provide an iron ion elution material for being integrated in a caisson and capable of effectively eluting bivalent iron ion (Fe2+) ingestible by photosynthetic organisms (chlorophyll and agar) into sea. ! SOLUTION: There is provided an iron ion elution material composed of a carbon material, an iron material containing metallic iron, and a floating body, wherein the carbon material and the metallic iron are at least partly made contact with each other. ! COPYRIGHT: (C)2014,JPO&INPIT