To provide a temperature responsive biodegradable gel which is used in a medical material such as a drug delivery system and an environmentally adaptable material, shows a swelling/shrinkage change in accordance with the change in temperature, has good biodegradability, and its constituting compound (crosslinked product).

The crosslinked product is obtained by crosslinking respective carboxyls of both of a copolymer of N-isopropyl acrylamide with acrylic acid and/or methacrylic acid and a polyamino acid which is safe and biodegradable and/or a mixture thereof and/or a salt thereof with a crosslinking agent such as an epoxy compound, and the temperature responsive biodegradable gel is obtained by dipping the crosslinked product in water.