PURPOSE: To simplify the structure and to reduce the power consumption by using all data pulses as synchronizing pulses and setting the time width of each pulse to a small value.

CONSTITUTION: While only transmission-side synchronizing pulses S are generated but transmission-side data pulses Dt are not generated, only a current I1 passing the first current setting resistance 6 drives a light emitting diode 4, and the diode 4 emits light with a middle intensity. When transmission-side data pulses Dt are generated, a current I2 passing the second current setting resistance 10 is added to said current I1, and the diode 4 emits light with a high intensity. Consequently, when synchronizing pulses St and data pulses Dt are inputted to the transmission side in accordance with signal "1010", the light emitting diode 4 emits light while changing the intensity. When the light of the light emitting diode 4 is received by a photodiode 16 in a receiver 3, its resistance value is changed in inverse proportion to the intensity of the received light, and the potential at a voltage dividing point P is changed in direct proportion to the intensity of the light.