NOVELTY - Polymeric fluorescent substance (I) showing fluorescence in the solid state and having a polystyrene reduced number average molecular weight of 103-108, comprising one or more phenyl repeating units (1a) and one or more aryl repeating units (2a), where the total amount of repeating units (1a) is 10-90 mol.%, and the total amount of repeating units (1a) and (2a) is 60 mol.% or more, is new.    USE - (I) is useful in a polymer light emitting device, which is useful: in a flat light source, a segment display, and a dot matrix display; and as a back-light in a liquid crystal display (all claimed). (I) is useful as a light-emitting material, charge transporting material, dye for laser, material for organic solar cell, organic semiconductor for organic transistor, material for conductive thin film, light source in curved or plane form for illumination, and optical material. The display devices are useful as a display of a computer, television, portable terminal, portable telephone, car navigation and view finder of a video camera.    ADVANTAGE - (I) exhibits: strong fluorescence; excellent charge transporting property; and high fluorescent quantum yield. (I) provides a high performance polymer light emitting device which can be driven at low voltage with high electroluminescent efficiency.    DETAILED DESCRIPTION - Polymeric fluorescent substance (I) showing fluorescence in the solid state and having a polystyrene reduced number average molecular weight of 103-108, comprising one or more phenyl repeating units of formula (1a) and one or more aryl repeating units of formula (-Ar1-) (2a), where the total amount of repeating units (1a) is 10-90 mol.%, and the total amount of repeating units (1a) and (2a) is 60 mol.% or more, is new.    R1-R4 = H or a substituent, where at least one of R1-R4 is alkoxy, aryl (substituted with alkoxy), aryloxy or arylalkoxy;    Ar1 = amine group of formula (6a); either    R8-R10 = 1-20C alkyl, 1-20C alkoxy, 1-20C alkylthio, 1-60C alkylsilyl, 1-40C alkylamino, 6-60C aryl, 6-60C aryloxy, 6-60C arylsilyl, 6-60C arylamino, 7-60C arylalkyl, 7-60C arylalkoxy, 7-60C arylalkylsilyl, 7-60C arylalkylamino, 8-60C arylalkenyl, 8-60C arylalkynyl, monovalent 4-60C heterocyclic group or CN; or    two or more of R8-R10 = ring, when one or more of R8-R10 is a group containing an alkyl chain, one or more of a methyl group, methylene group and methine group constituting the alkyl chain may be substituted with a group containing a heteroatom;    i, j = 0-4;    k = 0-5; and    h = 1-2.    An INDEPENDENT CLAIM is included for a polymer light emitting device comprising a pair of electrodes composed of an anode and a cathode at least one of which is transparent or semitransparent and at least one light emitting layer comprising (I) disposed between the electrodes.