A 61-year-old man, who had been diagnosed with pleuritis 5 months earlier, was admitted to our hospital to determine if a tumor shadow that appeared in his right lower lung field on March 2002 was a localized pleural mesothelioma. Although a CT-guided lung biopsy was performed, no definite diagnosis was made. However, because the tumor shadow continued to increase in size, we could not rule out the possibility of a malignant thoracic tumor, and performed video-assisted thoracoscopic surgery (VATS). Histological examination of the surgically resected tissue led to a diagnosis of thoracic actinomycosis in the main component of the pleura. Because the recurrence of pulmonary actinomycosis was also suspected after surgical treatment, penicillin G was administered intravenously and afterward amoxicillin was administered intraorally. Subsequently, the patient's clinical status improved. We considered a case of thoracic actinomycosis that was suspected to have spread directly from the lung to the chest wall with complicating pleural effusion and remained with organization because there was a pulmonary infiltration shadow in the right upper lobe on chest CT at the first admission.