PURPOSE: To enable uniform maintenance of the intensity distribution of signal light at an output end by forming the core part of a mixer having a high refractive index into a aquare column and covering the remaining four faces except the two faces to be connected with fiber arrays by clad parts consisting of glass having a low refractive index.

CONSTITUTION: A core part 11 of a mixer 18 having a high refractive index is made to a square column and the remaining four faces except the two faces to be connected with fiber arrays are covered by the 2nd clad parts 17 consisting of glass having a low refractive index so that the side faces of the core parts are not exposed to air. Joint surfaces 19 indicate the joint surfaces between the core parts 11 and the clad parts 12 and joint surfaces 20 indicate the joint surfaces between the conventional mixer and the 2nd clad parts. The exposure of the side walls of the core parts 11 is obviated by such constitution and therefore dew condensation is obviated, the condition for total reflection is maintained and the signal light having a uniform intensity distribution is obtd. from the exit end.