Microstomia is a term used to describe a small oral aperture. Most of the reported cases are caused by scar contracture after facial trauma, burn injury, and tumor excision. We experienced a rare case of microstomia in a patient with antilaminin 332 mucous membrane pemphigoid, which was an acquired autoimmune disease and showed blisters and erosive lesions mainly on the mucous membranes. The patient had recurrent aphthous stomatitis and presented microstomia caused by scar contracture of oral mucosa. We surgically corrected microstomia by 5-flap Z-plasty for commissuroplasty and 2 Z-plasty of both upper and lower lips for an enlargement of oral aperture. The patient could achieve an enough oral aperture and was satisfied with the result. There was no recurrence of microstomia for 2 years.