A polymer treating method has the steps of: reacting a polymer compound with a reaction agent in a reaction vessel to generate a polymer treatment product; discharging the polymer treatment product containing the reaction agent from the reaction vessel; depressurizing the polymer treatment product; introducing the polymer treatment product into a degassing extruder; separating the reaction agent from the polymer treatment product through a vent box that is connected to upstream of the degassing extruder and that has a volume of equal to or more than that of the reaction vessel; and extruding the polymer treatment product from the degassing extruder.