A 22-year-old woman, who had undergone intracardiac repair for pulmonary valve atresia with a ventricular septal defect when she was three years old, underwent right ventricular outflow tract reconstruction for re-stenosis. On the 22nd postoperative day, she was admitted to our hospital with a fever. She was diagnosed as having mediastinitis with infiltration of the lung contiguous to the site of the origin on contrast computed tomographic (CT) examination. She received cefazolin and vancomycin, and drainage was performed under general anesthesia the following day. There were many white blood cells but no bacteria in the pus on Gram staining ; on anaerobic culture and enriched broth culture, gram positive rods were detected which needed to be differentiated from Actinomyces spp. ; they were finally identified as Cutibacterium acnes by means of biochemical methods, 16S rRNA gene analysis, and time of flight mass spectrometry (TOF/MS). Cefazolin and vancomycin were changed to ampicillin ; after 5 weeks, infiltration of the lung lesion was still present. Ampicillin was then changed to oral amoxicillin and continued for a total of 10 weeks. She recovered completely. To our knowledge, there is no report of a case of mediastinitis caused by C. acnes with infiltration of the lung. We needed to differentiate the pathogen from contaminants on detecting C. acnes. In this case, we concluded that it was a pathogen because of the pure culture, and identification of C. acnes in several ways. Mediastinitis caused by C. acnes can infiltrate the lung. Surgery is not necessary for the management of the infiltrated lesion ; medical management with antibiotics is adequate. Its treatment is different from that of Actinomyces spp. The period of antibiotic administration needs to be determined by evaluating the CT image, because the period for treatment of the infiltrate lesion has not been established.