PURPOSE: To reduce the time delay of a communication by coupling a processor to an optical fiber cable loop via an optical node interface circuit provided with a switch to lead only required information from the loop to a node device.

CONSTITUTION: A signal level of control lines 24, 26 is brought into high level (H level) by a processor 10a in order to attain the frame synchronism at the node device at the initial setting and the time information of all time slots on an input optical fiber cable 1a is inputted to an photoelectric conversion circuit 6 through the 1st optical switch 20 for attaining the processing. Further, when a terminal or a telephone set makes no communication, the signal is led to the node and decoded at only a time corresponding to the time slot indicating a head frame bit and signal time slot, the signal is inserted again and transmitted to an output optical fiber 1b. The signal other than the said signal passes through an optical fiber 1c without being supplied to the node device.