It's one of the theme song for 恋がしたい 恋がしたい 恋がしたい (Koi ga s-h-i-tai, Koi ga s-h-i-tai, Koi ga s-h-i-tai). (name edited to work around censor)
Official Website
Info in English
The songs suit the drama well, but I couldn't find the song on www these days, which is strange for a timeless classic (I managed to find Rainbow Connection in IRC). Do anyone have a copy laying somewhere or managed to find it? Please PM me.
Thanks a lot.