A 14-year-old girl complained of back pain and weight loss. Laboratory data showed increased white blood cells, C-reactive protein, and erythrocyte sedimentation rate (ESR). Her chest radiograph and computed tomography (CT) scan showed a large tumor with satellite lesions in the right upper lung that had spread to the back muscle and invaded the left hemithorax through the mediastinum, suggesting malignancy. A CT-guided percutaneous tumor biopsy showed chronic inflammation with sulfur granules consistent with actinomycosis. It also showed the increase of α-smooth actin positive spindle-shaped cells, suggesting an inflammatory myofibroblastic tumor. She was treated with antibiotic tazobactam-piperacillin following cefozopran, and then amoxicillin. Her laboratory data improved, and the tumor disappeared. Therefore the diagnosis was confirmed as pulmonary actinomycosis, which is a disease frequently misdiagnosed and is quite rare in children. It should be kept in mind, however, when a pulmonary tumor is found with inflammatory manifestation in laboratory and pathological data, even in children.