We report a case of sinusitis caused by the displacement four interdental brushes into the maxillary sinus. A 73-year-old man was referred to our department to receive consultation for a strange feeling in the gingiva of the left maxillary first molar during hospitalization for an inguinal hernia operation. On intraoral examination, a spiral metal wire was confirmed in the distal side of left maxillary first molar. A computed tomographic scan showed a high density shadow and four metal-like images in the left maxillary sinus. We performed radical surgery for maxillary sinusitis and removed the foreign bodies under general anesthesia. The four foreign bodies were the distal ends of broken interdental brushes. Histopathological examinations revealed severe inflammatory cell infiltration of the maxillary mucosa and a perforation that was apparently caused by the interdental brush. After operation, we instructed the patient on the proper usage of interdental brushes, and there has been no sign of inflammation of the maxillary sinus. : displacement into maxillary sinus (上顎洞迷入) ,interdental brush (歯間ブラシ) 1) 函館五稜郭病院歯科口腔外科 (主任:宮澤政義診療部長) 2) 函館五稜郭病院パソロジーセンター (主任:池田 健センター長) 1)