A flame-retardant sealing material comprising a polyurethane foam formed from a polyurethane raw material containing a polyol component, an isocyanate, a water repellent, a flame retardant and the like, wherein the polyol component contains a hydroxyl-terminated prepolymer obtained by the reaction of a polyether polyol and an isocyanate, the water repellent is a polybutadiene polyol or the like, the flame retardant is (1) a non-halogen non-condensed phosphate ester or (2) a halogen-containing non-condensed phosphate ester, which are liquid at ordinary temperature, and the isocyanate index is 100 to 150.