Adenoid squamous cell carcinoma (ASCC) is an uncommon but well-recognized variant of squamous cell carcinoma that was first described by Lever in 1947. ASCC has been reported to originate in the sun-exposed skin of the head and neck and in other sites. An additional case of ASCC is reported here. The patient was a 64-year-old Japanese woman who requested examination of a reddish lesion on the left floor of the mouth. The biopsy material was diagnosed as squamous cell carcinoma. Clinical examination showed a well-circumscribed, 20 x 10 mm-sized lesion, which was categorized as cT2cN0cm 0. Tumor resection was therefore performed. Histologically, most parts of the lesion were conventional squamous cell carcinoma in situ, but the invasive part consisted of ASCC with gland-like or reticular appearance. The latter part was negative for mucin staining. Immunohistochemically, this lesion was positive for pancytokeratin, high-molecular-weight keratin, cytokeratin (CK) 7/8, CK19, E-cadherin and p53, but negative for vimentin, CK20, and S-100 protein. The Ki-67 labeling index was 50.3% in the ASCC part and 34.5% in the carcinoma in situ part. These findings and a review of the literature indicate that a gland-like feature of ASCC is associated with the loss of cell adhesion in the center of the cancer nests, and it can be confirmed simply by mucin staining to be neither an adenosquamous carcinoma nor ductal involvement of conventional squamous cell carcinoma.