To provide a back-fill for electrolytic protection which attains an electrolytic protection for a long period of time even under high resistant circumference, and is excellent in construction performance.

This back-fill for electrolytic protection contains metallic chloride and water-soluble micromolecule. It is preferable that the back-fill contains a polyhydroxy alcohol, further contains a crosslinking agent.It is preferable that the metallic chloride is magnesium chloride, the water-soluble macromolecule is polyvinyl alcohol, the polyhydroxy alcohol is glycerine, and the crosslinking agent is borax.