PURPOSE: To block an incoming from one station to another station without holding the CPU, etc., of the latter station, by releasing a trunk blocking state when detecting that the incoming is continuing for a specified time or longer, and regarding it as an actual incoming.

CONSTITUTION: A circuit 3 actuates an opposite station trunk only for a specified time by a trunk used for connection between telephone switchboards, and a circuit 7 detects that an incoming is continuing for a specified time; and a circuit 8 detects the continuation of incoming for a specified time or longer, and a circuit 9 holds a trunk blocking state. When a trunk blocking key 4 is put in operation, the opposite station trunk is actuated for the specified time and when the incoming continuing for the specified time is detected, it is regarded as a trunk blocking signal from the opposite station to hold the trunk blocking state. Further, when the continuation of incoming is continued for the specified time or longer is detected, the trunk blocking state is reset and the incoming is regarded as actual one.