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