PROBLEMThe present invention is directed to provide a thickening inhibitory agent for inhibiting peritoneal membrane thickening which enables inhibition/prevention or treatment of the peritoneal membrane thickening and which mitigates adverse effect, and a dialysis fluid comprising the aforementioned thickening inhibitory agent.SOLUTIONThe above-described problem is solved by means of an agent for inhibiting peritoneal membrane thickening, comprising as an effective ingredient a chromanol glycoside represented by the following chemical formula (1):(in this regard, however, R1, R2, R3, and R4 each represent a hydrogen atom or a lower alkyl group which may be the same as or different from each other； R5 represents a hydrogen atom, a lower alkyl group or a lower acyl group； X represents a mono-sugar residue or an oligo-sugar residue in which a hydrogen atom of hydroxyl group in sugar 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), or a dialysis fluid comprising the same.
