Boron isotopic ratios (¹¹B/¹⁰B) of 33 hot spring waters from central Japan were determined by mass spectrometry. The water samples can be divided into two groups based on their ¹¹B/¹⁰B values: one with higher ¹¹B/¹⁰B ratios (about 4.085-4.10), and another with lower ¹¹B/¹⁰B ratios (about 4.055-4.065). The ¹¹B/¹⁰B values are apparently independent of the pH, temperature, and chemical composition of the waters, but show a systematic regional variation. Hot spring samples from the Nasu, Kusatsu-Shirane, and Hakone areas belong to the higher ¹¹B/¹⁰B ratio group while those from the Takayu-Tsuchiyu, Myoko, and Shirahone areas belong to the lower ¹¹B/¹⁰B ratio group. This is explainable on the basis that the ¹¹B/¹⁰B value of a hot spring water is correlated with that of the boron source from which the aqueous boron is derived, and that the Izu-Bonin arc is strongly influenced by sea water which is enriched in ¹¹B.