@daisuke
1) remove the wall in the background, or try changing it for something that is more natural
2) add a skybox. Without a background picture and ambient lighting you'll not get a realistic picture.
3) good luck making a night shoot look real. That is a challenge I don't know much about yet, but every light is 10 times more important to get right.
@daisuke
4) try adding some textures to your scene. You can never achieve realism without them. (It doesn't even matter how much they fit, but our eyes hate surfaces without details.