A vaccine composition for birds comprising as an active ingredient a structure containing O-antigen derived from Gram-negative bacteria, provided that said structure does not contain a whole cell, and a process for preparing the same are provided. By using a structure containing O-antigen (e.g. lipopolysaccharide) derived from Gram-negative bacteria as an active ingredient in accordance with the present invention, alleviation of inoculation reaction and reduction in an amount of injection are attained as compared to the conventional whole-cells vaccine to thereby allow for the increase in the number of other antigens to be mixed therewith.
