PURPOSE: To obtain an optical signal having a large power and a narrow pulse width and a pulse interval, by switching sequentially a digital electric signal at each bit, stretching each bit in time, and inputting the result to a light source and synthesizing waves of irradiating outputs.

CONSTITUTION: A digital electric signal 11 is sequentially and switchingly into plural pieces e.g., three outputs at each bit with a signal switching device 12 and outputted, and the outputs 13a∼13c are supplied to communication light sources 14a∼14c. Further, the plural communication light sources 14a∼14c are output optical pulses 15a∼15c having a short irradiating time according respectively to the outputs 13a∼13c of the signal switching device 12. These optical pulses 15a∼15c are synthesized as optical signals at a synthesizer 16 and inputted to an optical fiber 18.