NOVELTY - A conductive polymer is obtained by copolymerizing phosphoric acid group containing unsaturated monomer and conjugated diene group liquid oligomer having ethylenically unsaturated bond(s), its hydrogenated product and/or vinyl aromatic-conjugated diene copolymer group liquid oligomer or its derivative having ethylenically unsaturated bond(s). The conjugated diene group liquid oligomer is chosen from butadiene oligomer, isoprene oligomer and its derivative. Vinyl aromatic-conjugated diene copolymer group liquid oligomer has vinyl aromatic and conjugated diene units.    USE - For forming solid polymer electrolyte of fuel cell (both claimed) e.g. direct methanol fuel cell primary battery, secondary battery, display element and sensor for signal transmission medium, solid capacitor and ion exchange membrane. Also used for antistatic agent, fog prevention material, modifier for paper and pulp, and coating agent.    ADVANTAGE - The conductive polymer has excellent solvent resistance, flame retardance and water resistance.    DETAILED DESCRIPTION - A conductive polymer is obtained by copolymerizing phosphoric acid group containing unsaturated monomer and at least one of conjugated diene group liquid oligomer having ethylenically unsaturated bond(s), its hydrogenated product and vinyl aromatic-conjugated diene copolymer group liquid oligomer or its derivative having ethylenically unsaturated bond(s). The conjugated diene group liquid oligomer is chosen from butadiene oligomer, isoprene oligomer and its derivative. Vinyl aromatic-conjugated diene copolymer group liquid oligomer has vinyl aromatic unit and conjugated diene unit. The phosphoric acid group containing unsaturated monomer is of formula: R1-C(=CH2)-C(=O)-O-(R2-O)n-P(=O)(OH)2.    R1=H or alkyl group;    R2=linear or branched butylene group;and    n=1-6.    INDEPENDENT CLAIMS are included for the following:    (1) solid polymer electrolyte film which has the above-mentioned conductive polymer;    (2) fuel cell which has solid polymer electrolyte film;    (3) flame retardant which comprises phosphoric acid group containing unsaturated monomer and at least one of (meth)acrylamide type monomer (CH2=C(R12)-C(=O)NH-R13), (meth)acrylonitrile, (meth)acrylic acid and N,N-dialkyl(meth)acrylamide;    (4) manufacture of phosphoric acid group containing unsaturated monomer which involves reacting hydroxyl butyl (alkyl)acrylate or hydroxy polyoxybutylene glycol (alkyl)acrylic (R1-C(=CH2)-C(=O)-O-(R2-O)n-H) and phosphoric anhydride, phosphorus oxy chloride, pyrophosphoric and/or polyphosphoric acid and hydrolyzing the obtained reaction product; and    (5) manufacture of flame retardant which involves performing radical polymerization of phosphoric acid group containing unsaturated monomer and at least one of (meth)acrylamide type monomer, (meth)acrylonitrile, (meth)acrylic acid and N,N-dialkyl(meth)acrylamide in a ketone solvent and/or alcohol solvent in the presence of an polymerization initiator, and isolating the obtained copolymer from solution followed by drying.