A 59-year-old woman, diagnosed with advanced rectal cancer, presented with a low-grade fever and dyspnea on exertion after the 2nd cycle of TAS-102. TAS-102 has promising efficacy in patients with metastatic colorectal cancer. A CT scan revealed mosaic patterns with bilateral ground-glass opacities. The drug lymphocyte stimulation test for TAS-102 was strongly positive and serum β-D glucan level was elevated. The clinical course was compatible with TAS-102-induced pneumonitis combined with pneumocystis pneumonia (PCP). We herein report a rare case of drug-induced pneumonitis in a patient receiving TAS-102 in combination with PCP.