To provide an inflation film having excellent mechanical strengths or seal strength which is suitably used as a garbage bag, a plastic bag, or the like even when the compounding ratio of the recovered PET resin being 60% or higher.

The recovered polyethylene terephthalate, three kinds of polyethylene-based resins having specific compositions and a thickner are mixed in a specified ratio to obtain a mixture, and then, the mixture is melt-mixed with a twin-screw extruder so as to obtain a resin composition as a pelletized raw material which is formed into a film by an inflation method.