To provide a heat-reserving material which is easy to process and has excellent heat-reserving properties.

The crosslinked heat-reserving material, which is a crosslinked product of a polymer or an oligomer, is constituted by the main chain portion X, the bonding portion Y, and the side chain Z represented by formula (1), and the side chain Z has as the main constituent a crystallizable unit which can crystallize. Since the side chain Z becomes amorphous or crystalline in the temperature range of -10 to 100°C with a large latent heat accompanied, the material is suitable as a heat-reserving material. It is crosslinked, so that it becomes a thermoplastic resin and can be molded. The molding thereof is less prone to lose its shape even when exposed to a temperature exceeding its melting point.