Even though, he has a girlfriend, he flirts with me like crazy at work?

I'm not really sure what your question is exactly, but I will try to offer some advice. You should attempt to tone down the relationship if he shows no sign of wanting to separate from his girlfriend, and you aren't even interested in him. Some people say flirting is harmless, but it could lead to something else at times. I would be pretty hot if I knew that my husband was flirting with someone else at work, so the girlfriend probably wouldn't like it either. If you do find yourself attracted to someone who flirts with you and has a girlfriend, be the bigger person and let him know that there is no chance for a relationship as long as he is with another woman.