Problem to be Solved: The present invention provides a novel mucosal adjuvant.Solution: The present invention provides a mucosal adjuvant composition containing at least a composition comprising molecules having a molecular weight in a range of 100 to 300 kDa obtained from cells or culture fluid of Bordetella bronchiseptica. Administration of the mucosal adjuvant composition to a non-human animal at a surface of the mucous membrane can enhance immunity at the surface of mucous membrane. Therefore preventive effects against trans-mucosal infection can be increased by administering an inactivated vaccine against trans-mucosal infection and the mucosal adjuvant composition to a non-human animal at a surface of mucous membrane. The present invention is effective for preventing trans-mucosal infections, including one or more infections of e.g., canine parainfluenza, canine adenovirus, canine coronavirus, canine parvovirus, canine distemper virus, canine herpesvirus, reovirus and pneumovirus.