The most aggressive foreign buyers of U.S. government debt see little reason to turn bearish after the biggest gains in longer-term Treasuries for two decades.

Japanese investors from Sumitomo Mitsui Trust Asset Management Co. to Mitsubishi UFJ Asset Management Co. say the lack of wage growth to spur U.S. inflation means owning the bonds still makes sense. Japan plowed $86 billion into Treasuries in the past 12 months, the most among the world's 10 biggest economies, data compiled by Bloomberg show.

Demand from the Asian nation lends credibility to the view Treasuries can rally further because the Japanese have contended with falling consumer prices and growth that's lagged behind the world for more than 15 years. Sumitomo Mitsui and Mitsubishi UFJ Asset were among investors that piled into longer-term Treasuries at the end of 2013, when bearishness reached a crescendo as most forecasters predicted losses.