The invention provides an additive for hydraulic compositions, which exhibits an excellent viscosity reducing effect on a hydraulic composition, for example, a hydraulic composition containing a slurry of water and hydraulic powder, and is excellent in operativeness when used as an aqueous solution. The additive for hydraulic compositions contains a phosphoric monoester having a specific group such as an alkyl ether group or a salt thereof (monoester) and a phosphoric diester having a specific group such as an alkyl ether group or a salt thereof (diester) in a monoester weight content of monoester/(monoester+diester) is 0.4 to 0.95 (diester includes a pyrodiester).