A microdebrider is increasingly used in endoscopic sinus surgery. Although it has many advantages over conventional instruments, it has been associated with severe complications. We treated a case of rupture of the left medial rectus muscle after use of a microdebrider during endoscopic sinus surgery in a 50 year-old female patient who complained of binocular diplopia and exotropia. The patient showed marked limitation on adduction and about 40 prism diopters of left exodeviation. The orbital computed tomography showed a bony defect at the left medial orbital wall, and injury of the medial rectus muscle. The exodeviation was corrected after ophthalmologic surgery. We report a case of the rupture of the medial rectus muscle after use of a microdebrider during endoscopic sinus surgery and review the medical literature.