The present invention relates to a fused aminodihydrothiazine derivative of formula (I): wherein R is hydrogen or C1-6alkyl, optionally substituted by one to five halogen atoms; n is 0, 1, 2 or 3; Ar is phenyl or a 5- or 6-membered heteroaromatic group containing 1, 2 or 3 N atoms, which Ar is optionally substituted by one to three substituents selected from hal, hydroxyl, —CN, C1-6alkyl, C2-3alkenyl, C2-3alkynyl, C1-6alkoxy, C3-6cycloalkoxy and pyrazine, where C1-6alkyl and C1-6alkoxy are optionally substituted by one to three halogen atoms; 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.