We explained about reports of fecal bacteria that could be biomarkers related to the efficacy of immune checkpoint inhibitor therapy with anti-PD-1/PD-L1 antibody against melanoma, non-small cell lung carcinoma (NSCLC), urothelial and renal cell carcinoma (UTC/RCC). It has been reported that the proportion of bacteria in the family Ruminococcaseae and Faecalibacterium affects the efficacy of immune checkpoint inhibitor therapy as biomarkers against melanoma, and Akkermansia muchiniphila as a biomarker against NSCLC and UTC/ RCC. However, it is unlikely that these intestinal bacteria can be applied to all carcinomas, and the mechanism of antitumor effects in these bacteria has not yet been fully elucidated.