Muscle relaxants and their reverse drugs should be carefully administered to patients with acute polymyositis and/or dermatomyositis. However, the use of these drugs in controlled polymyositis and/or dermatomyositis is controversial. This case report describes the use of rocuronium and sugammadex in a 27-year-old female patient with controlled polymyositis who was scheduled for minor oral surgery under general anesthesia. General anesthesia was induced rapidly, and 0.66 mg/kg of rocuronium was administered prior to nasotracheal intubation. No additional muscle relaxants were administered during the surgery. At the end of surgery, approximately 2 hours after the rocuronium was administered, her train-of-four (TOF) ratio was still 49%. A dose of 3.3 mg/kg of sugammadex was administered, and it took 12 minutes for the TOF ratio to exceed 90%. The prolonged duration of muscle relaxation in patients with polymyositis may be due to a decrease in skeletal muscle and capillary volume. The slow onset of sugammadex may be caused by slow diffusion of rocuronium from the neuromuscular junction. Patients with polymyositis require close perioperative neuromuscular function monitoring, regardless of their disease control status.