It is Miho Fukuhara's 2nd single, Himawari. The song is auto-biographic and talks about the death of her first childhood love. The PV moves around this fact too. It is depressing and really sad. I may sound like an emotional, but I cry every time I watch this.
If you like the song, here's a PV RIP:
http://www.ahsforum.com/forum/showth...62#post1536562