Japanese plugs are identical to North American plugs (with the two upright blades) except they never have the third, round hole. That's the ground - it makes the electrical outlet safer.

If you look at all the other outlets, they all have a ground as well (except for, it seems, the Russian one). Usually it's the middle hole, but on the French one, it's the silver pin sticking out, and on the German/Korean one, it's on the sides of the plug.