PROBLEM TO BE SOLVED: To provide an optical demultiplexer/multiplexer of high demultiplexing efficiency and multiplexing efficiency.
SOLUTION: The intra-surface hetero-structure two-dimensional photonic crystal periodically arranged with holes 32 with a period varying for each of respective forbidden band regions 301, 302, etc., is formed with a waveguide 33 passing the entire forbidden band region by linearly lacking the holes 32 and is formed with dot defects 341, 342, etc., by linearly lacking the three holes 32 by each of the respective forbidden band regions. The light passing the dot defects out of the light of the prescribed frequencies propagated through the waveguide from a light leading out/in section 36 and demultiplexed from the prescribed dot defects is not included in the waveguide transmission band 51 of the forbidden band region to which the frequency adjoins and is therefore reflected in the boundaries 351 and 352 of the forbidden band regions and is introduced into the prescribed dot defects. As a result, the demultiplexing efficiency of the light of the prescribed frequency taken out of the dot defects is improved. The light of the prescribed frequency multiplexed from the dot defects is increased in the intensity of the light arriving at the section 36 by the reflection and the multiplexing efficiency is improved.