A case of severe sleep bruxism（SB） treated with an occlusal appliance which restrains lateral movement by steep canine guidance is reported. The patient was a female in her 20 years who visited our hospital with a chief complaint of grinding sound of the teeth. Based on the clinical diagnosis of SB, treatment was initially performed using a stabilization appliance with a general form. However, no reduction in the number of SB waveforms was observed in the SB assessment by electromyography during sleep. On the other hand, when an appliance with restrained lateral motion by steep canine guidance was applied, the number of SB waveforms was significantly reduced. Even after long-term use of the appliance, there were no symptoms in teeth, periodontal tissue, temporomandibular joint, masticatory muscles, etc., and a good treatment course was obtained. From the treatment experience of this case, it is suggested that SB can be significantly reduced depending on the morphology of the occlusal appliance. Currently, however, there is no evidence that this form of appliance will be effective in all SB cases. Objective assessment of the effect using sleep electromyography needs to be performed in order to accurately determine the appropriateness of continuation of use. It is also considered necessary to carefully and regularly follow up on the condition of teeth, periodontal tissue, temporomandibular joint and masticatory muscles.