This paper describes Japanese ablaut from the historical point of view.　The ablaut is said to be a morphological phenomenon which occurred in ancient Japanese.　For
example, the vowel differences of /a/ and /e/ are seen in the words amagumo ‘rain cloud’ and amefuri　‘rainfall’; /u/ and /i/ in kamumiya ‘Shinto shrine’ and kamiwaza ‘work of God’; and also /o/ and
/i/ in konoha ‘leaves of trees’ and kinobori ‘tree climbing’. These vowel changes of compounds are due to the strength of the boundary between two words,　that is, morphemes.　When two morphemes construct a strong linking compound, a low vowel /a/ and back vowels /u, o/ are pronounced, and when two morphemes construct a weak linking compound,　front vowels, high front /i/ and mid front /e/ are pronounced.　We say that strong linking　compounds have a morpheme boundary between two morphological elements and weak linking compounds　have a word boundary between them.　These ablaut phenomena are explained by the front and back movements of the tongue in word-final positions.