To reduce clipping distortion and nonlinear distortion generated by electrooptcal conversion and to maintain high transmission quality by separating the level of input signals by a threshold value, performing the electrooptical conversion of respective electric signals by different electrooptical converters and then performing optical multiplexing.

The inputted electric signals are segment by a switch 1. A threshold level is set to the input electric signals, the electric signals higher than the threshold level are segmented to an electrooptical converter 2 and the electric signals less that the threshold value are segmented to an electrooptical converter 3 respectively. Further, the bias current of the electrooptical converter 2 is set near a threshold current by a bias circuit 4, and the bias current of the electrooptical converter 3 is set to a value larger than the threshold current, so as not to generate clipping by the bias circuit 5. Then, respective optical signals are multiplexed in an optical multiplexer 6 and transmitted by an optical fiber 7.