A previously healthy 73-year-old man was hospitalized with left complicated effusion and a consolidation in the left upper lung. He underwent a chest tube insertion and was treated with clindamycin but the consolidation remained after the treatment. We subsequently performed flexible bronchoscopy but it was impossible to make a diagnosis. Three months later, the consolidation had worsened so we performed another bronchoscopy. Finally, we were able to diagnose the consolidation as pulmonary actinomycosis, and to treat the condition appropriately. Pulmonary actinomycosis is a rare and difficult condition to diagnose. There are many conditions with similar clinical features, such as tuberculosis, fungal infections, lung abscesses, and lung malignancy. Respiratory physicians should consider the possibility of pulmonary actinomycosis when investigating patients with persistent pulmonary infiltrations. Early diagnosis and correct treatment may lead to a good prognosis and prevent unnecessary surgery.