This sustained-release topically administered agent includes: a compound having a pharmacological action； a crosslinkable polysaccharide derivative in which at least one active ester group capable of reacting with an active hydrogen-containing group is introduced into a polysaccharide side chain, the crosslinkable polysaccharide being capable of forming a crosslinked product by covalent bonding of an active ester group and an active hydrogen-containing group； and a pH regulator for establishing pH conditions to form a biodegradable gel containing the compound at the time of mixing. The sustained-release topically administered agent forms a biodegradable gel containing a compound at the time of mixing, and the pH of the resulting biodegradable gel is 5 or higher.
