1) In a RS485 multidrop network, to use a circuit_ground connection among the slaves as well as the master seems to be a must. 2) But my RS232/485 convertor (plugged in to a PC)has no GND output that supposed to be connected to the questioned network. It has only four output pin which two of them (T+ and T-)are to be used in my case. 3)So my question is, can I use the RS232's GND pin (pin 7) as connected to the common GND in RS485 network?

1) not necessarily. If the ground that exist at the nodes is no more different than the common mode rejection capability of the transcievers you can omit it. (I do not recommend doing so, but in many cases you have to - example two nodes, each connected to power ground, with a different ground potential, can fry your ground wire).
2) that is normal
3) see 1)

Erik

PS if you have grounding problems Maxim has some transcievers that can handle 60V common mode. They are a bit pricey, but making it work is more important than price.