The purpose of the present invention is to provide: a polypeptide containing an Fc region of which the activity of binding to both of genetic polymorphism types H and R of Fc?RIIa is maintained or reduced compared with that of a parent polypeptide and of which the activity of binding to Fc?RIIb is enhanced compared with that of the parent polypeptide； a pharmaceutical composition containing the polypeptide； a therapeutic or prophylactic agent for immune/inflammatory diseases, which comprises the pharmaceutical composition； processes for producing those products； and a method for maintaining or reducing the activity of binding to both of the above-mentioned genetic polymorphism types of Fc?RIIa or enhancing the activity of binding to Fc?RIIb. Specifically, it is found that a polypeptide which contains an antibody Fc region that contains a mutation produced by substituting Pro located at 238th position as numbered in accordance with the EU numbering method by Asp and a mutation produced by substituting Leu located at 328th position as numbered in accordance with the EU numbering method by Glu is enhanced with respect to the activity of binding to Fc?RIIb and is maintained or reduced with respect to the activity of binding to both of genetic polymorphism types H and R of Fc?RIIa. It is also found that a polypeptide which contains an antibody Fc region that contains a mutation produced by substituting Pro located at 238th position as numbered in accordance with the EU numbering method by Asp and another type of mutation is enhanced with respect to the activity of binding to Fc?RIIb and is maintained or reduced with respect to the activity of binding to both of genetic polymorphism types H and R of Fc?RIIa.
