A cement admixture for concrete using a polycarboxylate based water reducing agent that can produce light concrete with little slump loss and good workability that prevents dragging and the like and having no material separation, and a cement composition using the cement admixture are provided. A cement admixture for concrete using a polycarboxylate based water reducing agent, comprising bentonites (one or more selected from a group comprising bentonite, acidic white clay and active white clay are preferable) and polyvinyl alcohol, or the cement admixture further compounding saccharides. A cement composition, wherein cement, a polycarboxylate based water reducing agent, bentonites and polyvinyl alcohol are main components, or the cement composition, further compounding saccharides. Preferably, the cement composition, wherein the polycarboxylate based water reducing agent is 0.3 to 5 parts based on 100 parts of cement, bentonites are 0.1 to 10 parts, polyvinyl alcohol is 0.03 to 1 part and saccharides are 0.01 to 0.2 part.