The present invention provides a novel therapeutic strategy using an inhibitor targeting PD-1/PD-L1. A pharmaceutical composition which comprises a COX-2 inhibitor and is administered before, after or simultaneously with the administration of an inhibitor targeting PD-1/PD-L1. A potentiator for the immunostimulatory effect of an inhibitor targeting PD-1/PD-L1, which comprises a COX-2 inhibitor.