What do you think my teacher is thinking ?

It's very common for people to have a crush on a teacher at some point in their academic career and it usually passes with time.
As for your specific circumstance, I have to agree with ItDuz in that your feeling like he feels the same way may just be wishful thinking on your end. Without getting too over-complicated, sometimes we project our feelings onto others, making even mundane conversations and reactions seem significant and meaningful. I hope you're just reading too much into it. On the off-chance that you're not and he truly is flirting with you in a serious manner, there are more than a few ethical problems with it, most of which are obvious (against rules, he's married, imbalance of power, etc). You didn't specify, but I assume you're in university, not high school. Either way, absolutely nothing good can come of this situation if either one of you were to "make a move," so to speak.
The only advice I can really give you is to try to keep your mind off it. Whether it's with friends, just studying or maybe even signing up for an interesting extracurricular where you could try to meet guys that are more appropriate. I realize it probably seems hard to do now, but you'll eventually forget about it and move on.