It is an object of the invention to provide a method for producing a modified polymer compound and a production apparatus of a modified polymer capable of producing a modified polymer compound with a high purity in a short time, and such a modified polymer compound.The invention provides a method for producing a modified polymer compound, which comprises chemically modifying a polymer compound in a supercritical fluid or a high temperature and high pressure fluid, said modified polymer compound being composed of monomer units obtainable by chemically modifying part of or all of monomer units composed of the polymer compound before modification reaction.