To form a decorative pattern on a concrete product according to an aggregate exposed finish method by washing by using a cement hardening retarder capable of suppressing elution with moisture and sustaining the high ability to retard the hardening.

This cement hardening retarder comprises a polymer having units of a cement hardening retarding component hydrolyzable by actions of an alkali and is produced by introducing the units of the cement hardening retarding component hydrolyzable by the actions of the alkali into a main chain or a side chain of the polymer through ester bonds or acid amide bonds. The polymer having the hardening retarding component in the side chain is roughly classified into (1a) a homopolymer of a polymerizable monomer having the units of the hardening retarding component (a hydroxy-carboxylic acid, etc.) or a copolymer of the polymerizable monomer and at least either one of a copolymerizable monomer and a copolymerizable polymer (an unsaturated polyester) and (1b) a polymer in which the component having the ability to retard the hardening is bound to a reactive group such as carboxyl group in the side chain of the polymer through the ester bond. The polymer in which the hardening retarding component is introduced into the main chain may be a polyester containing the component having the ability to retard the hardening and at least two hydroxyl groups or carboxyl groups as a diol component or a dicarboxylic acid component. The hardening retarder may be a powdery or a granular material, a film or a sheet.