To provide a dispersing agent for alumina cement which is capable of imparting high fluidity, adequate pot life, strong exhibiting property, and high strength to an alumina cement blended substance.

The dispersing agent for alumina cement imparting excellent fluidity, adequate pot life, strength exhibiting property and high strength to the blended substance having the alumina cement as a hardener can be obtained when a copolymer having a specific range of an average number of addition mols of an alkylene oxide group in a side chain is used in a copolymer of a polyalkylene glycol vinyl ether and a maleic anhydride and/or its hydrolysis product.