PURPOSE: To suppress noise components in an optical transmission system and perform signal transmission of high reliability by the constitution consisting in photocoupling the input side and output side of a light delay element to one output terminal and input terminal, respectively, of a photomixing and branching element.

CONSTITUTION: A mixing and branching element 10 performing mixing and branching of light signals has at least two input terminals of optical fibers A, D and two output terminals of optical fibers B, C; the input signal (x) being inputted to the optical fiber A and the output signal (y) being outputted to the optical fiber B. The optical fibers C, D are photocoupled with a delay element 11 and the spreading, etc. of the pulse width of the ouput waveform (y) may be controlled by changing the branching rate of the mixing and branching element 10. Since the pulse width of noise components is usually narrow, taking a delay amount long has the effect of decreasing the peak values of respective impulses of the noise components and averaging them in time, and this enables the communication of high reliability to be realized.