The present invention aims to provide completely biodegradable and biocompatible sustained-release carriers for proteins or peptides, which allow encapsulation of the proteins or peptides at high encapsulation rates without inhibiting their biological activity. The present invention provides a method for preparing a sustained-release carrier, wherein in a solution in the presence of a protein or a peptide, a hyaluronic acid derivative having an unsaturated bond(s) or a mercapto group(s) introduced into hyaluronic acid is chemically crosslinked with a mercapto group-containing compound or an unsaturated bond-containing compound, respectively, to give a hydrogel. The method of the present invention enables efficient encapsulation of proteins or peptides while retaining their biological activity.