The gut(intestinal)microbiota is said to number about 1,000 species and about 100 trillion, and recent studies have reported that there is an interaction between cancer and the gut microbiota. It has been elucidated that certain gut microbiota affects the occurrence and risk of cancer, and in gastric cancer, an association with Helicobacter Pylori infection has been reported. Studies on multiple cancer types have also reported a correlation between the gut microbiome and the efficacy and toxicity of cancer immunotherapy. The gut microbiome has attracted attention as a promising biomarker for predicting the efficacy of immunotherapy, and interventional trials have been conducted to verify that it increases the efficacy of immunotherapy. Biomarker studies of immunotherapy and gut microbiome in advanced gastric cancer have reported associations between gastric cancer-specific gut microbiota and therapeutic efficacy and toxicity.