A 43-year-old man was referred to our hospital with symptoms of blood-stained sputum, low-grade fever, and left thoracic pain that had begun a few weeks earlier. A chest X-ray and thoracic computed tomography (CT) showed air-space consolidation with a cavity in the left upper lung. The patient was diagnosed with a lung abscess, and after four weeks of treatment with amoxicillin/clavulanate, his symptoms subsided. A chest X-ray performed at that time seemed to indicate that the lesion had disappeared. However, one month after the completion of treatment, the patient experienced a flare-up of respiratory symptoms. Five months after the initial treatment, a thoracic CT revealed a 2-mm fungus ball-like nodule in the residual-cavity area. Under diagnosis with a recurrence of lung abscess, a subsequent second course of amoxicillin/clavulanate over four months failed to control the patient's symptoms. Furthermore, the fungus ball-like nodule in the cavity was gradually increasing in size. Nine months after the first course of treatment, video-assisted thoracic surgery (VATS) was performed, and a resected specimen from the left upper lobe confirmed that the size of the cavity was 15 mm; moreover, it contained sulfur granules. Thus he was diagnosed with pulmonary actinomycosis. Only two cases of this rare radiological pattern of pulmonary actinomycosis with a fungus ball-like nodule within a cavity have ever been reported. Also, this is the first report showing the long-term generating process of a fungus ball-like nodule within a small cavity on follow-up thoracic CT, which by use of VATS biopsy was later identified as sulfur granules with pathological characteristic findings of a cavity wall.