The original disk space was 20 GiB, so windows keep asking me that they need a space. I found the web site http://jonmifsud.com/blog/increase-virtualbox-disk-size/. And follow what he did, and got the 50 GiB disk space.
After restarting Win 7,
Computer -> [Right mouse] management (G) -> Disk management -> Right mouse on [C:] (one can see the empty space after [C:] -> Volume expand (X) -> And following their explanation.
See what I got. (Korean Windows)