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 A ß production inhibitory effect or a BACE1 inhibitory effect and is useful as a prophylactic or therapeutic agent for a neurodegenerative disease caused by A ß and typified by Alzheimer type dementia.