The present invention is to provide a method for producing a copolymer for a cement admixture, which enables to easily produce a blend of a plurality of copolymers with different monomer compositions in one polymerization operation, and is superior in water reducing performance and/or slump retention ability. Disclosed is a method for producing a copolymer for a cement admixture, which comprises a step of polymerizing monomer components comprising unsaturated polyalkylene glycol ether-based monomer (A) and unsaturated organic acid-based monomer (B), wherein the unsaturated polyalkylene glycol ether-based monomer (A) is added into a reactor in advance and the unsaturated organic acid-based monomer (B) is added thereto with an addition rate thereof changed at least one time.