PURPOSE: To enable the private branch of exchange to cope with the occurrence of fault congestion in a way that a central controller is not loaded and the user feels no inconvenience by providing a relay control means to send a congestion wait tone such as a busy tone to a channel and the communication is awaited within a prescribed time.

CONSTITUTION: The private branch of exchange is provided with a congestion detection means 1 detecting the occurrence of a fault congestion state and starting congestion control and a relay control means 2 sending a congestion wait tone such as a busy tone to a channel and a line controller 1 is formed to await communication within a prescribed time. Thus, when calls of a prescribed number or over take place within a prescribed time, the congestion control is attained without giving a load to a central controller and the congestion is early restored.