The present invention provides long-acting sustained-release formulations of drugs such as proteins or peptides, which are injectable, completely biodegradable and safe, as well as ensuring efficient encapsulation of the drugs such as proteins or peptides without inhibiting their biological activity. It is possible to achieve injectable sustained-release formulations that ensure efficient encapsulation and long-term sustained release of drugs such as proteins or peptides while retaining their biological activity, when a solution containing a drug and a polysaccharide derivative such as hyaluronic acid having a crosslinkable functional group(s) or a salt thereof is dehydrated in microparticulate form starting from a dilute state, where crosslinking proceeds slowly, to reach a concentration which facilitates crosslinking, to thereby cause crosslinking reaction during concentration, so that the drug is encapsulated into the crosslinked polysaccharide to give drug-carrying microparticles.
