To provide a compatibilizer for polymer blend, having grafts of monomer B subjected to graft-polymerization up to the center of a polymer, capable of finely mixing together incompatible polymers having different components by polymer blend and usable without solidification even at high temperature.

This compatibilizer G' for polymer blend is obtained by graft polymerization of a polymer A having different components (e.g. a polyolefin polymer) with a monomer B (e.g. a carboxylic acid-based monomer such as acrylic acid or methacrylic acid), wherein, in the case the graft chain of the monomer B permeates into the polymer to a depth H thereof, the polymer A is a film having ≤2H thickness or a particle or fiber having ≤2H diameter, and the compatibilizer has such a configuration that the monomer B is graft- polymerized to the polymer A or a configuration that the carboxylic acid group of the graft chain of the graft-polymerized monomer B is derivatized (amidated or esterified).