PURPOSE: To prevent invasion of noise into the transmitter by providing a relay used to switch a supply source of a DC power supply for a transmission section from a common external power supply to an internal DC power supply through contacts based on the establishment of the internal DC power supply to the transmitter.

CONSTITUTION: In the transmitter TM inputting/outputting a transmission/ reception optical signal to/from optical cables F1, F2 via an electrooptic converter 3 and a photoelectric converter 4, when no external AC power supply 10 is supplied, since an internal power supply Vi+ is not arisen, a relay 5 used to select an external DC power supply 02 to the transmission section is deenergized. In this state, when voltages V+, V- of the transmission use external power supply 02 are supplied, the power supply is fed to a power supply line VT in the transmission section and the transmission circuit is activated. When the AC power supply 01 is supplied externally, the internal power supply Vi+ is arisen, the relay 5 is operated, its contact is closed and the internal power supply Vi+ is supplied to the transmission section internal power supply line VT and the transmission section is isolated from the transmission use external power supply 02.