Provided are: a new therapeutic strategy by using an inhibitor that targets PD-1/PD-L1; a pharmaceutical composition which contains a COX-2 inhibitor and is administered at any period of time prior to, subsequent to, or at the same time of the administering of the inhibitor targeting PD-1/PD-L1; and an immunostimulating effect enhancer which is for the inhibitor targeting PD-1/PD-L1 and contains a COX-2 inhibitor.