Hi
I want to ask that why is that my toilet seat discharges smell all the time and I feel as if it is the smell of the sewage system of the street. I've noticed that there isn't any toilet gas drain pipe outside of the toilet room. Is it the reason for that bad smell?

Are you sure the smell's coming from the seat and not the base of the bowl? How old is your house? There should be a vent off the lavatory that vents your toilet. If you don't see it on the roof then it's been revented back in the attic to the main vent. It's more likely that the wax seal has ruptured and the sewer gas's coming from there.
Check the wax seal and let me know what you find. Good luck, tom

I agree with Tom. If you seat was the source of the odor you would see a some human waste. The sewage smell is likley coming from the wax seal but odor are great for migrating around a room.