The present invention provides a powdery dispersant for hydraulic compositions, which is excellent in pumping before drying and in pressure resistance and humidity resistance, which comprises one or more copolymers obtained by polymerizing a specific vinyl monomer (1) such as an ethylenically unsaturated carboxylic acid derivative having a polyoxyalkylene group with a specific vinyl monomer (b) such as (meth)acrylic acid etc., wherein the average number of C_2-4 oxyalkylene groups or oxystyrene groups added is 45 to 150, (a)/[(a)+(b)]100 ranges from 15 to 45 (mol-%), and at least a part of the copolymers are monovalent or polyvalent metal salts.