Disclosed are: a thickening-inhibiting agent for inhibiting thickening of the peritoneal membrane by which thickening of the peritoneal membrane can be inhibited, prevented or treated with relieved side effects; and a dialysis solution which contains this thickening-inhibiting agent. Specifically disclosed are: an agent for inhibiting thickening of the peritoneal membrane, which contains, as the active ingredient, a chromanol glycoside represented by chemical formula (1); and a dialysis solution containing the same. In chemical formula (1), R 1 , R 2 , R 3 and R 4 are either the same or different and each represents a hydrogen atom or a lower alkyl group; R 3 represents a hydrogen atom, a lower alkyl group or a lower acyl group; X represents a monosaccharide residue or an oligosaccharide residue, in which the hydrogen atom in a hydroxyl group in each saccharide residue may be substituted by a lower alkyl group or a lower acyl group; n is an integer of 0 to 6; and m is an integer of 1 to 6.