The present invention aims to develop a method capable of evaluating T cell immune function. A method for evaluating T cell immune function including detecting and quantifying soluble PD-L1 having a binding ability to PD-1, and an ELISA system used for detecting and quantifying bsPD-L1, including a carrier with a solid-phased PD-1 protein, a means for reacting the carrier and a test sample, and a means for detecting and quantifying soluble PD-L1 bound to the carrier as bsPD-L1.