PURPOSE: To provide a switching request signal detection system by which the continuity of the state change of a switching request signal can be improved, the sate change of the switching request signal can be detected, and the processing load of a CPU can be reduced, at the time of executing a switching control by a line terminating device which terminates a present line and a preliminary line.

CONSTITUTION: This system is equipped with a switching request signal protecting circuit 1, a switching request signal status register 2 which holds the output signal state of the switching request signal protecting circuit 1, a comparator circuit 3 which compares the output of the circuit 1 with the output of the register 2, and interruption status register 4 which sets a corresponding bit according to the compared result of the comparator circuit 3, and operates an interruption to a CPU. Moreover, the CPU reads the data of the registers 2 and 4 through a bus transceiver 5, identifies which switching request signal is changed, and immediately executes the switching control of a unit corresponding to the changed switching request signal.