The present invention relates to a method of manufacturing an ionomer resin, comprising the steps of: (i) adding a poor solvent to a crude ionomer resin solution comprising a (meth)acrylic acid unit (A), a neutralized (meth)acrylic acid unit (B) and an ethylene unit (C) to allow a granular resin with a peak top particle size of from 50 to 700 μm to be precipitated; and (ii) washing the precipitated granular resin with a washing solution; wherein the total amount of the unit (A) and the unit (B) is from 6 to 10 mol % based on the entire monomeric units constituting the crude ionomer resin.