two different expressions both are grammatical correct. but they are expressing two different meaning. like the first one, it means you plan to do sth, but now you have not do it right now. it maybe happen in the future. the second sentence is one that express you are doing sth,, like you are now traveling to japan,,maybe right now you are just on a plane flying to japan.

You can also say, "I'm off to Japan tomorrow".

I'm going to go to Japan means you are going there soon (next week, next month) I call this tense 'Immediate Future'. I'm going to Japan means you are going there (you are in the plane, or at the airport, in an hour, tonight, tomorrow) I call this tense 'Present Continuous'. I will go to Japan (I don't know when, but I'll go some day, it is uncertain, though I want to go).

"am going to" = will I'm going to go to Japan = I will go to Japan. (after this, tomorrow or next week, next month, etc.) "am going" = refers to the action now. I'm going to Japan. (maybe you are in the plane or at the airport already)