A flame retardant union fabric obtained by combining (A) 30 to 70% by weight of a fiber comprising as a main component a flame retardant halogen-containing fiber made of a composition comprising 100 parts by weight of an acrylic copolymer of 30 to 70% by weight of acrylonitrile, 30 to 70% by weight of a halogen-containing vinyl monomer and 0 to 10% by weight of a vinyl monomer copolymerizable with them, 10 to 30 parts by weight of an antimony compound and 8 to 30 parts by weight of a zinc stannate compound, with (B) 70 to 30% by weight of a cellulosic fiber. The flame retardant union fabric shows a high flame resistance which passes the M1 class of NF P 92-503 burning test in France even after the post-treatment.