The present invention relates to a hydraulic composition dispersant containing a polycarboxylic acid-based polymer A having a carboxylic acid group and an oxyalkylene group and/or an oxystyrene group such as polymers obtained by copolymerizing a specified monomer 1 such as an ethylenic unsaturated carboxylic acid derivative having a polyoxyalkylene group with a specified monomer 2 such as a (meth)acrylic acid, and a polymer B obtained by copolymerizing the monomer 1, a monophosphate-based monomer 3 and a diphosphate-based monomer 4.