三線

Just read the R&R Newsmaker interview, and Miya confirmed that it was indeed him who played the Okinawan Sanshin in 優しい歌! :)
And it was composed during the World Tour Extra in Okinawa. :) :)
I like how he described what the song was all about. :) :) :)

Someone uploaded the performance of 25時の憂鬱 during 69Days live in Quattro on 3 Oct. It is WAY cool:
25時の憂鬱 - Shibuya Quattro, 06/10/03

Yes imagine watching it live. Imagine those sounds echoing through the livehouse, enveloping you completely. Pure b.l.i.s.s.