The present invention relates to a fused aminodihydrothiazine derivative of formula (I): wherein X is hydrogen or fluorine； A is CH or N； Y is methyl, ethyl, monofluoromethyl, difluoromethyl, trifluoromethyl, difluoroethyl, methoxy, ethoxy, methoxymethyl or &mdash；C&equiv；N； and pharmaceutically acceptable salts thereof； which compound has an A&bgr； production inhibitory effect or a BACE1 inhibitory effect and is useful as a prophylactic or therapeutic agent for a neurodegenerative disease caused by A&bgr； and typified by Alzheimer-type dementia.
