I submitted a paper to a journal, after 2 months and some days of review I get a rejection with a referee report "I find the paper not suitable for the journal", without any further comments and without explaining the reason of the rejection. However the journal publish articles in the topic of my paper! I find it unfair. How could I react to it?
Submit somewhere else.
The accept/reject decision is made by the editor, based on the recommendations of the referee(s). In this case, the editor felt that the referee's opinion of unsuitability, even without any explanation, was sufficient to reject. Maybe the editor felt that the unsuitability was self-evident upon inspection. Maybe the editor trusts the referee's judgment so implicitly that they didn't feel the need to ask for any explanation. Maybe there were other referee reports that the editor decided not to share with you. We don't really know. But in any case, whether this decision was made well or poorly, I don't think there is anything you could say that would be likely to change the editor's mind.
You could of course decide not to submit to that journal any more, and you could share your experience with colleagues. You could also try to have your paper looked at informally by colleagues in the field, to see if it has severe flaws that you haven't noticed. You could even complain to the editor-in-chief that you don't think the editor is giving due consideration to the papers they handle (though this if the editor-in-chief agrees that the paper is self-evidently unsuitable, this will only annoy them).
But don't try to pursue publishing this paper in this journal - it's a waste of time.
I agree with previous answers, but I like to add another explanation. A couple of times I had to review manuscripts that where very preliminary. In Spanish we say "está muy verde" (as if it was an immature fruit). At the time of writing my concerns I had realized that if the reviewers provide too much guide for further work, in fact his/her contribution will deserve to be listed as one of the authors, since he/she played a role in the study design, analysis or whatever. In this scenario a bunch of lazy (lost) researchers just can throw incomplete pieces of investigation to some of the many journals and harvest a lot of "free" guidelines, mentorship, name it as you like.
Although I have never done it, the best option for a reviewer that suspect that it is the case, is to reject without many details (to the authors), but writing his/her concerns to the Editor. In this way the Editor have the option of disclose the reasons of the rejection if he/she think that it is fair.
Looking at your original question and at some of your comments here, I wonder if it wouldn't be worth having a native speaker read through your paper, and make sure it's not rife with grammatical gaffes.
no my paper a good one
No, my paper is a good one.
Not a high level journal in my fields.
Not a high-level journal in my field.
I submitted a paper to a journal, after 2 months and some days of review I get a reject
would be better as:
I submitted a paper to a journal; after more than 2 months of review I received a rejection
You don't mention any co-author, so I assume you wrote the paper yourself. It could be that your findings are solid, but there are so many grammatical errors that some reviewers thought the work was unfit for publication. Perhaps they felt awkward saying that outright, so they didn't comment any further.