A novel adjuvant composition is provided that is excellent in safety and convenience as compared to the conventional adjuvant such as Alum. An adjuvant composition comprising citrullines, which are water soluble substance present in the living body, and/or a salt thereof; a vaccine composition comprising said adjuvant composition and an antigen; a process for preparing said adjuvant composition and said vaccine composition; and a method for administering said adjuvant composition and said vaccine composition are provided.