Recently, immune checkpoint inhibitors(ICI)has been developed rapidly as a novel cancer therapy. In particular, the number of worldwide clinical trials using anti-PD-1/anti-PD-L1monoclonal antibodies(mAb)are ongoing in various types of cancer. On the other hands, the response rate of anti-PD-1/anti-PD-L1 mAb monotherapy has been around 10-40% in various types of cancer, therefore, the development of biomarker for them are urgently needed. The following conditions in the tumor microenvironment are necessary for anti-PD-1/anti-PD-L1mAb therapy; the coexpression of HLA class Ⅰand PDL1 on tumor cells, and the existence of CTL around tumor cells. In this review, we discussed the mechanism of PD-L1 expres- sion and the development of biomarker of anti-PD-1/anti-PD-L1 mAb from the viewpoint of IFN-γ that can induce those conditions in the tumor microenvironment.