A method for producing a polymeric fluorescent substance characterized by comprising the step of treating with an acid a polymeric fluorescent substance that is fluorescent in a solid state and has a polystyrene equivalent number average molecular weight of 1.times.10.sup.4 to 1.times.10.sup.8. A method for producing a polymeric fluorescent substance characterized by comprising the steps of treating with an acid a polymeric fluorescent substance that is fluorescent in a solid state and has a polystyrene equivalent number average molecular weight of 1.times.10.sup.4 to 1.times.10.sup.8 and treating it with an alkali. A method for producing a polymeric fluorescent substance characterized by comprising the steps of treating with an acid a polymeric fluorescent substance that is fluorescent in a solid state and has a polystyrene equivalent number average molecular weight of 1.times.10.sup.4 to 1.times.10.sup.8, treating it with an alkali and finally treating it with a substance that does not contain an acid or an alkali.