To obtain the subject new compound characterized by the structure in which the carboxy and hydroxy groups are crosslinked with each other by the lactone or intramolecular ester bond, useful for sanitary or surgical products, cosmetics, medicines, foods, or the like which use biodegradable plastics.

This compound is a crosslinked carboxylic acid type polysaccharide (except hyaluronic acid), wherein at least the first part of the carboxy group in the polysaccharide is crosslinked with the hydroxy group in the same polysaccharide molecule and/or the hydroxy group in a different polysaccharide molecule through the ester or lactone bond. This compound is obtained by treating a polysaccharide esterified with a free or salt type carboxy group and monovalent alcohol or polyalcohol (e.g. alginic acid, carboxymethyl cellulose and carboxymethyl chitin), and then heating the above mixture or irradiating it with ray (in particular, ultraviolet ray).