NOVELTY - Polymeric fluorescent substance (I) which emits a fluorescence in a solid state, has a number-average molecular weight of 103-108 in terms of polystyrene, comprising one or more repeating units (v) and (vi), is new, in which the amount of (v) is not less than 0.1 mol.% and not more than 20 mol.% and the total amount of repeating units (v) and (vi) is 50 mol.% or more.    USE - (I) is useful as a back-light in a polymer LED, which is useful for producing a flat light source, a segment display, a dot matrix display, and LCD (all claimed), where the display devices are useful as e.g. a display of a computer, television, portable terminal, portable telephone, car navigation or view finder of a video camera.    ADVANTAGE - (I) has strong fluorescence, and provides the polymer LED with lower voltage and high electroluminescent efficiency.    DETAILED DESCRIPTION - Polymeric fluorescent substance (I) which emits a fluorescence in a solid state, has a number-average molecular weight of 103-108 in terms of polystyrene, comprising one or more repeating units of formula ((X4-(-Ar9-(CR39=CR40)f)-Ar10)e) (v) and ((Ar12)c-(Ar11)-(CR51=CR52)d) (vi) is new, in which the amount of (v) is not less than 0.1 mol.% and not more than 20 mol.% and the total amount of repeating units (v) and (vi) is 50 mol.% or more.    Ar9, AR11 = 6-60C arylene or 4-60C heterocyclic group contained in the main chain;    Ar10, Ar12 = 5-20C alkyl or 7-60C arylalkyl;    X4 = -CR41=CR42-, -C=C-, -SiR43R44-, -NR45-, -CO-, -COO-, -O-CO- or -SO2-;    e, c = 1-4;    Ar9 = -X4-Ar10;    R39-R45, R51, R52 = H, 1-20C alkyl, 6-60C aryl, 4-60C heterocyclic groups or CN, when the substituent of Ar10, R39, R45, Ar12, R52 and R51 includes an alkyl chain, and ends of many alkyl groups may bond to form a ring, one or more -CH2- groups contained in this alkyl chain may be replaced with -O-, -S-, -CR46=CR47-, -C=C-, -SiR48R49-, -NR50-, -CO-, -CO-O-, -O-CO- or -SO2-;    R46-R50, R53-R57 = H, 1-20C alkyl, 6-60C aryl, 4-60C heterocyclic groups or CN, where one or more H contained in the repeating unit (v) or (vi) may also be replaced with halo comprising F, Cl or Br; and    f, d = 0 or 1.    An INDEPENDENT CLAIM is included for a polymer LED 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 disposed between the electrodes, where (I) is contained in the light emitting layer.