The object of the present invention resides in providing a method and instrument of polymer processing treatment, wherein a sudden pressure change due to expansion by gasification of a reactant when it is exhausted from a reacting extruder is given in a little amount to a molding machine for a treated substance, and a change in the amount of the material supplied to the molding machine and a variation in molding are little, and hence continuous and uniform molding is possible. That is, the method of the present invention resides in a method of polymer processing treatment, which includes the steps of: reacting a polymer with a reactant under mixing and stirring at a high temperature and a high pressure in the inside of a reacting extruder to produce the treated substance; introducing said treated substance and a remaining reactant into a reactant-separation tank to separate the treated substance and the reactant; and molding the separated treated substance.