It's big, but it is still believable in size. Salt water crocs can easily hit that size if they get old enough and sometimes go pretty far out into the ocean. To think one would go to Japan is not likely, just because Japan is really cold and they don't have the wildlife and river systems that large crocs like this prefer.
actually that's an extraordinary size, even Gustave (the contender for largest saltwater crocodile ever, though still unmeasured) is only estimated to be over 20'. there are very few recorded saltwater crocs that make it to 20'. the "crocodile" in the picture above would be 30', not to mention, it would have to swim across open ocean hundreds of miles to reach Japan from any country a saltwater crocodile is native to, and being a reptile, would more than likely freeze to death before reaching Japan if such a journey was made by a saltwater croc.
After a search i could not find a single account of a crocodile being that size, as a matter of fact the world record size one wasn't nearly this big. Here it is.
He did measure 20ft which is impressive, but this ****** is clearly pushing a solid 30ft or over. And it would appear there are no accounts of this monster anywhere. No records i can find at least. The croc you mentioned is still roughly 10 feet smaller than biggie smalls over here. So my point stands, this is fake. Or Very VERY new, so new that no one has reported on it yet. Which i doubt.