PURPOSE: To make a power line unnecessary by connecting a light transmitting device including a signal generating section, a light transmitter/etc. and a central controlling section only by fibers and using a solar battery as a power source of the light transmitting device.

CONSTITUTION: The light transmittter 3 of the light transmitting device amplifies digital signals, modulates a light emission element in terms of strength and sends to the central controlling section 10 through optical fibers 9. On the other hand, a switch for supplying power intermittently from a solar battery 4 and a battery 5 to a signal generating section 1, an interface 2 and the light emitter 3 and a clock that determines timing of opening and closing the switch are incorporated in the power source controlling section 6 of the light transmitting device. The period T0 of transmission is determined by the clock, and power is supplied for a time TS to send signals, and power supply is not made during T0-TS. and transmission is stopped. Though the clock is driven at all times, power consumption is very small.