Patent ID: 8757915

Claim:
An adapter unit comprising: a first adapter comprising: a holding portion provided with a pair of retention holes of rectangular cross-section for accommodating a pair of optical connectors; a first cylindrical portion having an internal space communicating with said pair of retention holes, said first cylindrical portion being connected to a front end of said holding portion; at least one key formed on an inner peripheral surface of said first cylindrical portion so as to extend along a central axis of said first cylindrical portion; and a pair of projecting portions formed on the inner peripheral surface of said first cylindrical portion; and a second adapter comprising: a second cylindrical portion which is insertable into said first cylindrical portion; a key groove formed in an outer peripheral surface of said second cylindrical portion, said key groove being adapted to be fitted to said key along the central axis of said first cylindrical portion; and a pair of receiving portions formed on the outer peripheral surface of said second cylindrical portion and being adapted to receive said pair of projecting portions; wherein: two corners of each of said retention holes are in contact with the inner peripheral surface of said first cylindrical portion; a front end of said key is located behind a front end of said first cylindrical Portion; said pair of projecting portions are positioned symmetrically about the central axis of said first cylindrical portion, each of said pair of projecting portions having a level surface and a receiving surface connected to the level surface; said level surface is flush with one inner surface of a corresponding one of said pair of retention holes; and said receiving surface is located between the front end of said key and the front end of said first cylindrical portion and receives a front end surface of said second cylindrical portion when said second cylindrical portion is inserted into said first cylindrical portion.