Infratemporal fossa cellulitis is rare and mostly occurs because of sinusitis and dental procedures. Furthermore, cellulitis caused by iatrogenic foreign bodies is very rare. A 28-year-old woman who had previously undergone cosmetic bimaxillary operation visited our hospital complaining of left facial swelling, oppressive pain, and nasal obstruction since 2 years. She had been attending another clinic, but despite having additional procedures and taking medications, her symptoms persisted. A subsequent operation was performed, during which we found a remnant surgical gauze from the previous operation, which was decomposed and trapped around the necrotic soft tissue and had eroded the bony structure around the pterygoid fossa. The material was successfully removed by endoscopic surgery, and the necrotic tissue was debrided. After the operation, all symptoms disappeared, and the patient was discharged without sequelae. During any procedure, surgeons must meticulously check for remnant material. Additionally, physicians must carefully note patient history and perform a physical examination, even in patients without serious symptoms. We report a case of advanced infratemporal fossa cellulitis due to remnant gauze material during a previous operation that was undetected.