A poultry producibility improver containing a polymannose having such a molecular weight distribution that the molecular weights ranging from 1.8x10 to 1.8x10 account for 70 % or above; a pultry producibility improver further containing a polyphenol; a poultry producibility improver furthermore containing defatted rice bran; and a method of improving the producibility of laying hen or chicken breed by using any of the above improvers. The invention permits the poultry producibility to be improved at a low cost.