Provided is a method of producing a nucleator masterbatch, by which a nucleator masterbatch capable of improving the transparency and the physical properties of an olefin resin can be produced. This method is a method of producing a masterbatch in which a nucleator is blended in an olefin polymer, the method being characterized by comprising the step of polymerizing an olefin monomer with incorporation of a nucleator component, which is dissolved in an organoaluminum compound or in an organoaluminum compound and an organic solvent, before or during polymerization of the olefin monomer such that the nucleator component is blended in an amount of 0.05 to 20 parts by mass with respect to 100 parts by mass of the olefin polymer obtained by the polymerization of the olefin monomer.