Is it impolite to ask a professor who was not my master's thesis advisor (but with similar research field) to help me to work on a research topic? I have finished my master's thesis and graduated from the university in which I used to study for my masters degree. Now I have two or three months free time and I am searching for a PhD position; in this period of time I want to work on research topic, I see two professors at that university, whose research fields are really interesting to me. Their research field is really near to that of my master's thesis advisor.

Is it considered impoliteness to my master's thesis advisor if I ask another professor in his department (with similar research field to him) to help me work on a research topic?
Not at all. You are not committed to doing your PhD with the same advisor. Even if you did want to work with the same advisor, he/she should encourage that you talk to others to hear out their ideas. I highly doubt your advisor would be offended to hear that you were interested in speaking to other professors about their research interests. After all, a PhD is a long road, and you need to be really happy with your advisor, and your work, to survive it. 

In fact, I think it looks positive, even to your advisor, knowing that you have multiple options. I once had a job interview where, upon disclosing that I was also interviewing at other places, that "they would be worried if I wasn't".