Dendritic cells (DCs) play a crucial role in maintaining the immune system. Though DC-based cancer immunotherapy has been suggested as a potential treatment for various kinds of malignancies, clinical efficacies are still insufficient in many human trials. To identify the causes of the low efficacies, we paid attention to their numbers and how they are activated. We proved that DCs' antitumor effect depends on their number and the way they are activated. We here established a possible breakthrough, a simple cytokine-based culture method to realize a log-scale order of functional murine/human DCs. Moreover, we demonstrated that DCs activated by replication-deficient recombinant Sendai virus (rSeV) were dramatically more effective than that seen in the use of current DC vaccine for immunotherapy against malignancies. Our study could overcome these problems and would improve treatment of malignancies.