To prevent the optical path deviation of the ferrule of an optical fixed attenuator attached to a multifiber adapter.

For the optical fixed attenuator 1, an adapter connection part 2 to be inserted to each of a plurality of insertion ports formed in a columnar shape on the multifiber adapter is formed on one end side and a plug connection part 3 to which a plug is to be inserted is formed on the other end side. The outer shell of the plug connection part 3 is formed by a casing 5 of roughly a square tube cross section. A long groove 12 is formed on an opposing surface in the casing 5 and thin guide plates 13 for guiding the plug to be inserted are formed on both sides of the long groove 12 so as to partially cover a plug inserting part.