A sustained-release topically administered agent includes a compound having a pharmacological action, a crosslinking polysaccharide derivative having at least one active ester group, introduced into a side chain of a polysaccharide, which is capable of reacting with an active hydrogen-containing group, and being capable of forming a crosslinked substance by a covalent bond between the active ester group and the active hydrogen-containing group； and a pH adjuster for adjusting a pH condition suitable for forming a biodegradable gel containing the compound when the sustained-release topically administered agent is mixed. The sustained-release topically administered agent forms the biodegradable gel containing the compound when the sustained-release topically administered agent is mixed. The pH of the biodegradable gel is not less than five.
