Hi there hirohiro, welcome to the forum.
Yes, you are right about Visit Visa. It can be tricky to get one.
However the cost of the visa is quite low so I guess well worth the risk.
Your g/f will need to prove that she has more compelling reasons to return home than reasons to stay in UK beyond the visa expiry.
Of course, other requirements need to be met such as financing the trip for the requested duration and reasons to visit etc.
Take a look at the application form and UKBA webpage Visiting the UK for a better understanding of the needed information.
Feel free to ask any additional questions here.
