A 33-year-old previously healthy man injured his gums and subsequently developed dyspnea and fever. A chest X-ray showed nodules and infiltrates in both lungs, and the patient was initially diagnosed with pneumonia and administered meropenem hydrate, although his symptoms did not improve. A blood culture identified Fusobacterium necrophorum, and thrombophlebitis in the internal jugular vein of the neck was observed on computed tomography and ultrasound scans. We replaced the meropenem with clindamycin, sulbactam/ ampicillin and metronidazole, and the patient's symptoms improved.