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Just as with the German and US government bond yields of the same maturity it is too early to call a bottom in Japan. Market action of the next few months will hopefully bring us more hints.
All markets trade at historically low levels, which we will probably not see again in our lifetime after bonds change trends.
Organization for Economic Co-operation and Development, Long-Term Government Bond Yields: 10-year: Main (Including Benchmark) for Japan© [IRLTLT01JPM156N], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/IRLTLT01JPM156N, September 15, 2017.