We herein report a case of pulmonary actinomycosis that was difficult to differentiate from lung cancer. The patient was a 53-year-old man who visited our hospital with a chief complaint of bloody sputum. Computed tomography (CT) showed a tumor in the anterior basal segment (S8) of the left lower lobe that contained a low density area, and because elevated C-reactive protein (CRP) was also observed, the patient was diagnosed with a lung abscess. As no improvements were seen after 4 months of antibiotic administration, and accumulation was observed at the same site on positron emission tomography (PET), the patient was diagnosed with lung cancer and underwent surgery. Following segmental resection, the patient was found to have pulmonary actinomycosis. It is necessary to consider pulmonary actinomycosis in patients with lung tumors.