Just to be clear, I would never use kreem in my tank... But it's an option that people have been using for a long time with (mixed) results (of course I think as eric mentioned, prep and application has a lot to do with results).

From the looks of the pic, I would do an inspection of the tank and see what the "damage" is first. I am guessing it's not that bad.

Not sure what the state of the fuel is in Japan, but in the US some one came up with the great idea of ethanol fuel (currently at 10%). It's absolute sh*t and has destroyed more then it's done good. I never put fuel in any of my bikes (or lawn equipment, and I have 5 acres of lawn to maintain, so I have a good sized fleet of lawn care equipment) without some form of ethanol stabilizer (Sta-bil) to help reduce the damage of crappy fuel.

I also drain my motorcycles carbs and tanks when not using them, but some people aren't willing to go through that trouble. Im just tired of rebuilding/replacing carbs so it's worth it to me.

Oh and if you have run pre-mix in your tank, rust shouldn't be that much of an issue, you should have a good coating of oil to help protect the inside of the tank.