It's weird, Toshi, I don't see any mention of Yamaha's guitar strings on the US (Yamaha) web site.It has good silk & steel strings.
Silk and steel strings were developed by Yamaha.
Yamaha calls them compound strings.MIJ.
Yamaha's compound strings are softer than Martin's silk light strings, and are much easier to play over long periods of time.
Please try once.