Outline of the case: We reported a case of severe awake bruxism (AB) with frequency of muscle activity during an awake state being far from normal but with amount of muscle activity during sleep being within the normal range. The patient was a female in her 80s who visited our hospital with chief complaint of shifting of mandible to the right and awareness of clenching teeth. After clinical examinations, conventional electromyography (EMG) was performed at the hospital, but involuntary muscle activity at rest were not distinctly observed. A conventional upper jaw oral appliance was set to stabilize mandibular position. Then, the upper and lower removable partial dentures and other necessary dental treatments were conducted, and the mandibular position became stable without feeling of shift of mandible to the right. However, the patient was still aware of clenching and complained pain of mucosa under the dentures when wearing for a long time. Therefore, using a wearable EMG device, EMG were examined during awakening and sleep in daily life. Results: The number of EMG episodes was extremely high during daytime awakening, while those during sleep was within normal range. Therefore, on the upper and lower jaws, we manufactured oral appliances with denture structure in missing teeth area so that the load on the ridge of missing teeth area at the time of clenching could be controlled during awakening. As a result, the pain of the jaw ridge disappeared, and the awareness of clenching was reduced. Conclusions: By using the wearable EMG device, the abnormal activities of the masticatory muscles during the daytime which was not found in the hospital was able to be detected. Based on the EMG findings, effective countermeasures were conducted. The present case demonstrated that severe AB can occur even in a patient who is not a sleep bruxer.