PURPOSE: To prevent a plurality of terminal devices from using the same storage area of a common memory at the same time, by inhibiting the use of common memory based on the request from a terminal device, when the storage area of the common memory requested with the terminal device is in use.

CONSTITUTION: To avoid use of a common storage area of a common memory 21 in an RAM20 by terminal devices 21-1W21-N, an inhibiting table 22 and a CPU11 storing the information recognized with a storage area relating to the usage state of the common memory 21 are provided. The CPU11 discriminates whether or not the information recognizing the area is stored in the table 22 when a usage request of a storage area of the memory 21 is given from a terminal device. As a result, when the discriminating information relating to the usage state is stored, the use of storage area is inhibited and if not, the use of the storage area is permitted and the information discriminating the area is stored in the table 22.