An object of the present invention is to provide a chondroitin sulfate having a decreased molecular weight which has utilization as an inhibitor of peritoneal disorder caused by long-term use of a peritoneal dialysis fluid containing glucose or a polysaccharide thereof as an osmotic agent, utilization as an osmotic agent in a peritoneal dialysis fluid, and the like. The chondroitin sulfate having a decreased molecular weight of the present invention as a means for achieving the object is characterized by having a weight average molecular weight of from 1000 to 20000 and containing a constituent disaccharide unit represented by the following structural formula in an amount of from 65 % to 100 % (molar ratio) of the total. - 4 �¢ GlcA² �¢ 1 - 3 �¢ GalNAc 6 �¢ S �¢ ² �¢ 1 - (wherein GlcA represents a D-glucuronic acid residue； GalNAc represents an N-acetyl-D-galactosamine residue； ²1-3 represents a ²1-3 glycosidic linkage； ²1-4 represents a ²1-4 glycosidic linkage； and (6S) indicates that position 6 of the monosaccharide residue is sulfated)
