PURPOSE: To improve crosstalk performance by inserting an optical filter into the optical fiber type demultiplexer/multiplexer.

CONSTITUTION: The optical fiber glass part 2 in the fixing part of the optical fiber type demultiplexer/multiplexer is fixed to a fixing member 4 by an adhesive 5. A groove 6 arriving from the fixing member 4 to at least the core part of the optical fiber is cut. The groove 6 may be cut down into the position where the optical fiber is cut. An optical filter is inserted into the cut groove 6 and is fixed by an optical adhesive material, etc. As a result, the optical fiber is provided with the light passage characteristic meeting the characteristic of the optical filter 7.