Provided is a method of producing a masterbatch, in which a reduction in the viscosity of a polyester resin is inhibited and thereby a metal salt compound can be incorporated in a large amount. This method is a method of producing a masterbatch that comprises 0.4 to 10 parts by mass of at least one metal salt compound selected from the group consisting of metal benzoates, metal aliphatic carboxylates, metal aromatic phosphates, metal salts of sulfonamide compounds and metal salts of sulfonimide compounds with respect to 100 parts by mass of a polyester resin, wherein when the polyester resin and the metal salt compound are fed to an extruder and kneaded, the cylinder temperature of the extruder is controlled in a range of (the melting point of the polyester resin—60° C.) to (the melting point of the polyester resin—10° C.).