A method of treating a polymer compound has the steps of: mixing and agitating the polymer compound and a chemical agent in an extruder while controlling the inside of the extruder to be kept at a high temperature and a high pressure to have a reaction mixture containing a polymer treated product; supplying the reaction mixture continuously into a high-pressure container disposed following the extruder and retaining the reaction mixture at the high temperature and high pressure for a predetermined time; then supplying the reaction mixture continuously to a chemical agent separator to separate the polymer treated product; and supplying the separated polymer treated product continuously to a molding means to form it into a molded material.