Abstract: In an optical communication set a local transmission equipment, in which a call switch is previously pressed, becomes a master side. The local transmission equipment at the other side automatically becomes a slave side in response to a burst digital signal transmitted from the master side. The master side equipment supervises the transmitting signal and the receiving signal, adds a signal for deciding a sync state in the transmitting signal, and transmits it to the slave side equipment. The slave side equipment detects the sync signal, controls the timing of its transmitting signal, and transmits the signal to the master side equipment, thereby making a communication state of two-way time division optical communication having a predetermined sync relationship. When the transmission call of either side is made, the relationship between the master side and the slave side is established.