The problem to be solved by the present disclosure is to provide a novel combination therapy using (S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)pyrrolidin-1-yl)prop-2-en-1-one or a salt thereof, the combination therapy exhibiting an excellent antitumor effect on cancer patients with resistance to immune checkpoint inhibitors. The present disclosure provides an antitumor agent comprising (S)-1-(3-(4-amino-3-((3,5-dimethoxyphenyl)ethynyl)-1H-pyrazolo[3,4-d]pyrimidin-1-yl)pyrrolidin-1-yl)prop-2-en-1-one or a salt thereof as an active ingredient, the antitumor agent being administered in combination with an immune checkpoint inhibitor, except for pembrolizumab, to a cancer patient with resistance to immune checkpoint inhibitors.