To provide a joint filling material for fire prevention used for filling a gap between structures in fireproof panel covering a seismic isolator with a laminated base-isolating rubber, which has satisfactory mounting workability and durability and excellent flexibility and fire resistant properties.

The joint filling material for fire prevention has flame retardant properties, thermally expands when a fire occurs, and the residue after burning has sufficient shape retainability and flexibility, by adding and mixing an expansive graphite, boric acid, and an inorganic filler into a rubber component containing a specific amount of a thermoplastic elastomer.