A private branch exchange comprises trunk circuits, communication highways connected to the trunk circuits for transmitting data of a time-divisional multiplexed channels, and a time switch for switching the communication highways. A communication highway is connected to a computer network via a circuit/packet converter. The computer network comprises branch networks to which the circuit/packet converter is connected, and a root network to which the branch networks are connected through a router/gateway. Each computer terminal has an extension number and IP/port addresses different for such communication functions as audio communication, data communication and the like. The circuit/packet converter has bearer circuits different for each communication function and allocates a specified bearer circuit to each time slot of the communication highway. When a call is originated, the circuit/packet converter controls the time slot of the communication highway such that a bearer circuit vacant and suitable for a call type is connected to the computer terminal. The bearer circuit interfaces data on the computer network with circuit data on the communication highway. Therefore, the computer terminal can be connected to the exchange through the communication highway and caused to function as a terminal connected to the private branch exchange for each of various communication function.