That truck has fire shooting out the back because it is a Yaki Imo truck and is roasting sweet potatoes. This one happened to be driving around Tokyo so you don't see the man standing outside his truck hawking them with a characteristic Yaki Imo cry. If you google Yaki imo you can probably find an audio file of it. The yaki imo truck is pretty similar in concept to ice cream trucks except that they serve something hot rather than cold and their product is more rustic and hand-made.
Roasted sweet potatoes are quite delicious. You can see a pile of the raw reddish/purple ones just in front of the oven (under the scale). They also are an illustration of the reverence that Japan has for food. I don't think in the US that they would allow this sort of truck with open flames driving around the streets, and even though Japan is in many ways more safety conscious than the US, they appear to make exceptions where food is involved.
I'll have more and better examples of some of the variety of Japanese food as I work my way through the photos I took in the last few weeks, so treat this one as just a snack or a warm-up
