A defoaming agent for cementitious compositions is provided, which has an excellent defoaming performance and does not segregate when mixed with a high performance AE water-reducing agent to form a single admixture or when diluted in water at the desired concentration, and which has excellent long term storage properties. The defoaming agent for cementitious compositions is obtained by mixing at least a polyethylene oxide derivative and a nonionic defoaming agent wherein the polyethylene oxide derivative has at one end a hydrophobic group with a branched structure and/or an unsaturated bond and at the other end an anionic group.