Immune checkpoint inhibitors(ICIs)are playing an increasingly important role in the treatment of cancer. In the field of lung cancer, ICIs are widely administered from primary therapy to maintenance therapy after chemoradiation for non-small cell lung cancer. However, excluding tumor proportion score(TPS)for PD-L1, no other biomarker has been reported to be clinically useful. While many biomarkers are being searched for, analysis of intestinal microbiota is attracting attention as a parameter that may reflect immune status. Research on the relationship between ICIs and gut microbiota has expanded worldwide after 2 reports in Science in 2015. In a study in which the gut microbiota of ICI-treated patients was transplanted into germ-free mice, enhanced antitumor effects were observed in the group that received gut microbiota from the response group, suggesting the possibility of stool transplantation. At the same time, when Akkermansia muciniphila, which is one of the mucin-degrading bacteria, was ingested by mice transplanted with non-responsive gut microbiota, a portion of tumor-infiltrating T cells increased on tumor localization, indicating the effect of changes in gut microbiota. In addition, there is a possibility that the anti-tumor effect may be enhanced by the effect of metabolites on immune cells in the blood rather than the gut microbiota itself, and the analysis of metabolites produced by bacteria is attracting attention. In our department, we have analyzed the intestinal microbiota of 25 non-small cell lung cancer patients treated with anti- PD-1 antibody. Although we have achieved diversity and identification of specific bacterial species, analysis of bacterial metabolites will be important in the future when considering the impact of the intestinal microbiota on immune cells. The gut microbiota is not only a biomarker for the treatment of ICIs, but also has the potential to create an immune state that facilitates the effects of ICI by changing the gut environment and metabolites.