The invention provides a miniature adapter for MPO plug which can make the adapter small to the limit so as to be the same size as a plug, can be mounted at a high density without any restriction in an arrangement place and can be manufactured with a simple structure at a low cost. An adapter main body is formed into a tubular shape by upper and lower wall portions which are approximately flush with upper and lower wall portions of a coupling, and left and right wall portions which are approximately flush with left and right wall portions of the coupling, and is provided in both front and rear ends of the left and right wall portions of the adapter main body with elastic locking portions which are provided so as to face to a side of plug insertion and removal opening portions.