A method of manufacturing an optical multiplexer/demultiplexer includes the steps of: detachably holding optical filters and a mirror respectively to one surface and to the other surface of a pair of mutually parallel surfaces of gripping jigs; gripping a block holder between the gripping jigs with interposition of adhesive layers so as to reflect the parallelism of the pair of the surfaces; heating the gripping jigs gripping the block holder; and cooling the gripping jigs with them gripping the block holder. The optical filters and the mirror are thereby bonded to the block holder in parallel to each other.