NOVELTY - A polymeric fluorescent substance showing fluorescence in the solid state and having a polystyrene reduced number-average molecular weight of 103-108, comprises repeating units of specified compounds.    USE - The inventive polymeric fluorescent substance is used as a light-emitting material and/or charge transporting material for a polymer light emitting device useful as back-light of a liquid crystal display device or a light source for illumination, and a display device, e.g. flat panel display such as a segment display and a dot matrix display (claimed). It can be used as a dye of laser, a material for organic solar cell, an organic semiconductor of organic transistor, and a material for conductive thin film.    ADVANTAGE - The invention exhibits strong fluorescence and/or excellent charge transporting property, and a high performance polymer light emitting diode than can be driven at low voltage with high efficiency.    DETAILED DESCRIPTION - A polymeric fluorescent substance showing fluorescence in the solid state and having a polystyrene reduced number-average molecular weight of 103-108, comprises repeating unit(s) of formulae (I) and Ar1. The total amount of repeating units of formula (I) is at least 10 mol% and at most 90 mol% based on the total amount of repeating units of formulae (I) and (II).    R1-R4 = H;    R1, R2, R3, and/or R4 = alkoxy, aryl with alkoxy, aryloxy, or arylalkoxy;    Ar1 = a different group from (I), and phenylene (having substituent(s) other than alkoxy, aryl substituted with alkoxy, aryloxy, and arylalkoxy), substituted stilbene (having on benzene ring, alkoxy, aryl substituted with alkoxy, aryloxy, and arylalkoxy), distilbene group, condensed polycyclic aromatic compound, condensed polycyclic heterocyclic compound, aromatic amine compound, or groups of formula -Ar2-Ar3-Ar4-;    Ar2, Ar3, Ar4 = arylene or divalent heterocyclic compound.    An INDEPENDENT CLAIM is included for a polymer light emitting device comprising a pair of electrodes containing anode and cathode at least one of which is (semi)transparent, and at least one light emitting layer containing the inventive polymeric fluorescent substance.