Recently, the analysis of microbiota has been of interest not only for the clarification of the molecular mechanisms of disease etiology, but also the discovery of novel strategies for treatment. Following the development of "next-generation" sequencing, novel areas have been discovered in microbiota; however, in oncology, the relationships between microbiota and cancer have not been fully clarified. In recent literature, surprisingly, detection of gut microbiota in tumor issue itself has been reported. Microbiota might play an important role in carcinogenesis. However, this phenomenon is not well understood, and research in this area has just begun. In the past five years, a paradigm shift has occurred in cancer treatment due to immunotherapy. Immunotherapy has made cure possible even in advanced cancer patients with not only melanoma but also non-small cell lung cancer and others. In this review, we discuss the mechanisms of novel immunotherapies, checkpoint inhibitors, and the relationship between microbiota and immunotherapy. It is of significance to clarify this relationship because it may lead to the discovery of predictive markers for immunotherapy and promote clinical efficacy. Finally, we also mention our activities in the construction of a big database for information on immunotherapy and microbiota, which may lead to excellent possibilities of discovering novel strategies for more effective cancer treatments, and may accelerate the alteration of cancers to the classification of chronic nonfatal disease.