53 years old female showed repeated ulceration of labial gingival mucosa at upper and lower anterior teeth, which was a partly desquamated and erythematous lesion. The lesion was slightly extended into vestibule and buccal mucosa in oral cavity, but the similar lesion was not found in other organs by medical inspection. The incisional biopsy including the border of the ulcerated mucosa and normal mucosa showed a severely inflamed mucosa, of which epithelium was gradually detached from the underlying conective tissue, so that it was diagnosed as a mucous membrane pemphigoid (MMP) pathologically. The epithelium was thinned, almost lost its rete pegs, and the basement membrane was completely distorted by the epithelial detachement. The inflammatory cell infiltration was mainly composed of small round cells and plasma cells. Immunohistochemistry was performed to know the expression of pathogenetic proteins using antisera of Igk, E-cadherin, laminin a5, elafin, and eIF5A. The basement membrane at the epithelial detachment was condensely positive for Igk, and the involved epithelium became atrophic but showed consistently positive reaction of matrix proteins and protein translation factor, i.e., E-cadherin, laminin a5, elafin, and eIF5A similar to the adjacent normal mucosa continuous to the MMP lesion. The Igk was also diffusely deposited on the basement membrane of nearby normal mucosa. Many plasma cells infiltrated around the lesion were strongly positive for Igk in their cytoplasms. Therefore, we suggest that the MMP be characterized by the deposition of Igk on the basement membrane of the detached epithelium in the absence of no other pathognomic changes of molecular events.