PURPOSE: To stop charging on a call terminating side by a mischievous telephone call by switching to the charging on a call originating side corresponding to the reception of an interruption signal from a call terminating side telephone set by a switchboard during a call by the charging on the call terminating side.

CONSTITUTION: The call is performed between the call terminating side telephone set 2 and a call originating side telephone set 3 by the charging on the call terminating side. In this case, when the line circuit 11 of the switchboard 1 receives the interruption signal from the telephone set 2, a CPU 17 converts a state to a call originating side charging call, stores a charge respectively in a charging frequency storage part 5 and a storage part 16 and connects the call. Thus, the charging on the call terminating side by the mischievous telephone call can be prevented from being performed.