The present invention provides an adjuvant for cancer antigen peptide vaccines and virus antigen peptides, containing a pertussis vaccine as a primary ingredient. The present invention also provides a therapeutic agent for a cancer or viral infectious disease, and a prophylactic agent for metastasis or recurrence of cancer or onset of virus-induced tumor, containing a cancer antigen peptide or virus antigen peptide and a pertussis vaccine. A pertussis vaccine that can be suitably used is a whole cell body pertussis vaccine. The agents of the present invention can be safely administered in a plurality of doses.