A flame-retardant urethane resin composition comprising: a polyisocyanate compound; a polyol compound; a trimerization catalyst; a foaming agent; a foam stabilizer; and an additive, wherein the trimerization catalyst is at least one kind selected from the group consisting of a nitrogen-containing aromatic compound, a carboxylic acid alkali metal salt, a tertiary ammonium salt, and a quaternary ammonium salt, and the additive comprising red phosphorus as an essential component in an amount within a range of 3.0 parts by weight to 18 parts by weight with respect to 100 parts by weight of the urethane resin and at least one other component, wherein the other component is: (i) at least a bromine-containing flame retardant, (ii) a combination of a bromine-containing flame retardant with one kind selected from the group consisting of a phosphoric acid ester, a phosphate-containing flame retardant, and a boron-containing flame retardant, (iii) a combination of a bromine-containing flame retardant with a phosphoric acid ester and a phosphate-containing flame retardant, or (iv) a combination of a bromine-containing flame retardant with a phosphoric acid ester, a phosphate-containing flame retardant and a boron-containing flame retardant, wherein the bromine-containing flame retardant is an aromatic bromine-containing flame retardant selected from the group consisting of hexabromobenzene, pentabromotoluene, hexabromobiphenyl, decabromobiphenyl, hexabromocyclodecane, decabromodiphenylether, octabromodiphenylether, hexabromodiphenylether, bis(pentabromophenoxy)ethane, ethylene bis(tetrabromophthalimide), brominated polycarbonate, brominated epoxy compounds, poly(brominated benzyl acrylate), brominated polyphenylene ether, brominated polystyrene, and crosslinked or non-crosslinked poly(methyl styrene). a polyisocyanate compound; a polyol compound; a trimerization catalyst; a foaming agent; a foam stabilizer; and an additive, wherein the trimerization catalyst is at least one kind selected from the group consisting of a nitrogen-containing aromatic compound, a carboxylic acid alkali metal salt, a tertiary ammonium salt, and a quaternary ammonium salt, and the additive comprising red phosphorus as an essential component in an amount within a range of 3.0 parts by weight to 18 parts by weight with respect to 100 parts by weight of the urethane resin and at least one other component, wherein the other component is: (i) at least a bromine-containing flame retardant, (ii) a combination of a bromine-containing flame retardant with one kind selected from the group consisting of a phosphoric acid ester, a phosphate-containing flame retardant, and a boron-containing flame retardant, (iii) a combination of a bromine-containing flame retardant with a phosphoric acid ester and a phosphate-containing flame retardant, or (iv) a combination of a bromine-containing flame retardant with a phosphoric acid ester, a phosphate-containing flame retardant and a boron-containing flame retardant, (i) at least a bromine-containing flame retardant, (ii) a combination of a bromine-containing flame retardant with one kind selected from the group consisting of a phosphoric acid ester, a phosphate-containing flame retardant, and a boron-containing flame retardant, (iii) a combination of a bromine-containing flame retardant with a phosphoric acid ester and a phosphate-containing flame retardant, or (iv) a combination of a bromine-containing flame retardant with a phosphoric acid ester, a phosphate-containing flame retardant and a boron-containing flame retardant, wherein the bromine-containing flame retardant is an aromatic bromine-containing flame retardant selected from the group consisting of hexabromobenzene, pentabromotoluene, hexabromobiphenyl, decabromobiphenyl, hexabromocyclodecane, decabromodiphenylether, octabromodiphenylether, hexabromodiphenylether, bis(pentabromophenoxy)ethane, ethylene bis(tetrabromophthalimide), brominated polycarbonate, brominated epoxy compounds, poly(brominated benzyl acrylate), brominated polyphenylene ether, brominated polystyrene, and crosslinked or non-crosslinked poly(methyl styrene).