This species resembles Ipomoea triloba, but the flower of the former is white.  This species propagates as a naturalized plant in Japan, too.  Photos taken by YY on 1975.10.1 (slide 034) and 1980.7.7 (slide 035).   
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1 x 1 (thumbnail images) : For indexing 2 x 2, 4 x 4, 8 x 8 : For viewing on monitor 8 x 8, 16 x 16 : For printing