To provide a method for manufacturing a copolymerized polyethylene terephthalate resin composition which is almost free from voids generated in the interface between a copolymerized polyethylene terephthalate resin and inactive particles by uniformly dispersing the inactive particles without allowing the presence of a coarse particle in the copolymerized polyethylene terephthalate resin and a method for manufacturing a copolymerized polyethylene terephthalate film, and the copolymerized polyethylene terephthalate film with excellent surface smoothness manufactured by the method.

This method comprises a first process to heat the copolymerized polyethylene terephthalate resin into a molten state, a second process to add the inactive particle to the molten copolymerized polyethylene terephthalate resin and a third process to knead the molten copolymerized polyethylene terephthalate resin with the inactive particle. When the inactive particle is added in the second process, a fine powder of polytrimethylene-2,6-naphthalate resin with an average particle diameter of 10 to 1,000 μm is added simultaneously with the inactive particle.

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