A study to evaluate the effect of metformin on the immune system was commenced in July 2014. Metformin is one of the most commonly prescribed drugs for type 2 diabetes, and previous studies have reported that metformin has an anti-tumor effect. The aim of this study is to evaluate the efficacy of metformin on the immune system in human cancer patients in vivo. The primary outcome parameter will be the rate change in the population of CD8＋ T cells, which produce multiple cytokines.