According to the present invention, a heterodimer protein composition that is composed of a first polypeptide comprising an immunoglobulin heavy chain constant region and a second polypeptide comprising CL-Fc prepared by fusion of an immunoglobulin light chain constant region (CL) and Fc region, and also has a deletion or substitution of Cys residues involved in disulfide bonds between CL and CH1, a purification method of the protein composition thereof, a preparation method of the protein composition thereof, a DNA and a vector encoding the protein composition thereof are provided.