The song is "Why (Keep your head down)" by DBSK aka TVXQ aka Tohoshinki. God, how many names they have... This is the Korean version, in my opinion it's much better than the Japanese one. And I just love it~

Click to view

Really. I was watching some Tohoshinki videoclips on YouTube and I found this:

( Jumon - MIROTIC - Toshoshinki )