The present invention relates to a method for improving the solubility of a physiologically active protein or peptide compared to that of a physiologically active protein or peptide which is not conjugated to an immunoglobulin Fc fragment, in which the method comprises conjugating the physiologically active protein or peptide to an immunoglobulin Fc fragment; and a composition for improving the solubility of a physiologically active protein or peptide, comprising an immunoglobulin Fc fragment, in which the composition improves the solubility compared to a composition without an immunoglobulin Fc fragment..