A mobile communication system, a control server and a mobile communication method, in which a particular communication cost separate from a normal communication cost can be set to communications between mobile communication terminals locating in a specified area, and a conventional infrastructure can be effectively utilized by using a new add-on device onto the existing switched mobile network without its large improvement. In response to a request to carry out a communication from one mobile communication terminal to another mobile communication terminal using a particular accounting system, the control server determines whether of not the mobile communication terminals locate within the respective specified areas to send allowance of the communication using the particular accounting system to the mobile communication terminal on a sending side.