PURPOSE: To improve the reliability of the whole of the system by connecting light emission elements in parallel and disconnecting automatically one light emission from the system when the light emission element has a level lowered and operating the other light emission element and connecting this element to the system.

CONSTITUTION: Input signals applied to terminal 1 are inputted to driving transistors Tr3 and 6 of light emission elements 4 and 7 connected in parallel through two-distribution amplifier circuit 2. Tr3 is always conductive and causes element 4 to emit light, and Tr6 is always cut off by emitter-side relay 8. The optical output from element 4 is connected to transmission line 31 through optical fiber 32 and optical switch 35. A part of the light from element 4 is converted photoelectrically by light receiving element 5 and is amplified by 10 and is smoothed by 12 into a DC to drive trigger circuit 16, and a voltage is applied to electrode 29 of the optical switch and Tr9 of relay 8 drive. When the level of element 4 is lowered, relay 8 is released by Tr9, and Tr6 is made conductive to cause element 7 to emit light, and optical switch 35 is switched to switch transmission line 31 to optical fiber 33.