A 59-year-old man was referred to our hospital due to an abnormal thoracic shadow. Contrast-enhanced chest computed tomography (CT) and magnetic resonance imaging revealed a posterior mediastinal tumor with supreme intercostal artery invasion at the apex of the chest cavity. Based on these imaging results, a tumor of neurogenic origin was suspected. Tissue specimens collected by CT-guided percutaneous needle biopsy revealed xanthogranulomatous tissue containing parasite eggs. Since the patient had a history of ingestion of Japanese freshwater crabs which are known as an intermediate host, we performed a serum antibody test against Paragonimus westermani using microplate enzyme-linked immunosorbent assay (ELISA), which was positive. Finally, Paragonimiasis westermani was diagnosed. Four months after administration of praziquantel, the posterior mediastinal tumor had shrunk and P. westermani-specific antibody titers had decreased.