Just one question

I just want to know, that does peoples in japan understand very good english? I want travel to Japan one day in future, and want to be sure that I get along with english in Japan, because I speak just few words japanese.
Please, answer soon and thank you.

I just want to know, that does peoples in japan understand very good english?

No. But most people understand some English. Enough to make it easy for a traveler without Japanese language skills to travel in Japan, especially if you keep to the beaten path.

Most people in Japan have learned at least a little English in school. However most have little opportunity to use it conversationally and are somewhat reluctant to use it for fear of making errors or appearing foolish. Most can understand better than they can speak, and read better than they can write. My wife's niece, a college grad, has studied English for a number of years, writes creditably, but is still hesitant to use English in conversation. She really does very well, but is afraid of making errors. When you think about it, it's very similar to the situation English speakers find themselves in when they've studied, for instance, French in school, but have little opportunity to use it.

At any time, there are thousands of tourists who don't speak any Japanese travelling independently in Japan and thy all manage somehow. My parents have been here twice and travelled around by themselves; they were fine.

Names of train stations are also written in English and increasingly stations, train changes and other information are announced on the train in English, in Tokyo at least. Many other signs are in English and a lot of restaurants have menus in English or photos of the food so you can point.

You will be fine. In the meantime, why not start learning some basic Japanese?