Japan is one of the richest country in the world. I think it is wrong.

Japan is one of the richest countries in the world. It is correct. Is it?

"Japan is one of the richest country in the world." I think that's wrong.

"Japan is one of the richest countries in the world?" Is that correct?

Yes. There are a number of rich countries in the world, and Japan is one of them.

Dear friend,
I am not giving importance to the sentence.
I hope you answered grammaticaly.
I am not asking whether Japan is a richest country or not.
I am giving importance "country" and "countries".

Quite simple.

In English, countries is plural and country is singular.

Japan is one of the richest countries in the world. CORRECT
(There are many countries in this world and Japan is richest of them.)
Here we will not use country . Since you added in the world we are talking about the countries in the whole world.