A flame-retardant hydraulic fluid having excellent suitability for coagulating sedimentation in wastewater treatment. The flame-retardant hydraulic fluid comprises a polybasic organic acid/polyether compound adduct obtained by reacting a polyether compound having a hydroxy value of 2 to 300 mg-KOH/g and having 1 to 8 hydroxy groups with an unsaturated polybasic organic acid by radical reaction.
