The present inventors worked on elucidating the mechanism of antigen cross-presentation by dendritic cells, and revealed that foreign antigens taken up in dendritic cells are degraded by proteasomes after undergoing polyubiquitination. Based on the novel finding that polyubiquitination is involved in cross-presentation, the promotion of polyubiquitination was attempted by a number of methods, and the promotion of antigen presentation was confirmed. These methods enable the production of dendritic cells effective for inducing CTL activation.