PURPOSE: To clear a system automatically by resetting a switch netwark by erasing the contents of a temporary memory unit in the key telephone system by a central processor, when a service request from each button telephone set is not sent within a fixed time.

CONSTITUTION: Several main wires 1 are connected to respective columns of the matrix of switch network 3 by way of dial signal sending and trunk circuit 2, and respective rows of the matrix are also connected to sound circuit 24 through sound interface 4 which supplies a direct current to button telephone sets 10. This network 3 is controlled by CPU5 with an incorporated microprocessor, and CPU5 is connected to a memory circuit composed of ROM6 and RAM7, and input-output interface circuit 8, which is connected to each telephone set 10 through data interface 9. Then, when any service request from each telephone set 10 is not sent from CPU 5 within a fixed time, contents of memories in interface 9 and RAM7 are cleared and network 3 is released, thereby putting the system in a stand-by state.