<p>The present invention relates to a fused aminodihydrothiazine derivative of formula (I): wherein X is hydrogen or fluorine; R is monofluoromethyl or difluoromethyl; and pharmaceutically acceptable salts thereof; which compound has an ?ß production inhibitory effect or a BACE1 inhibitory effect and is useful as a prophylactic or therapeutic agent for a neurodegenerative disease caused by ?ß and typified by Alzheimer-type dementia.</p>
