PURPOSE: To prevent erroneous connection of optical fibers even if the connection is made by an ordinary user, by making parts of optical fiber connectors other than their fitting parts different in shape from each other by at least one or more.

CONSTITUTION: Parts of optical fiber connectors other than their fitting parts are made different in shape from each other in, for example, such a way that projections 10A and 11A are added to a plug-side connector 5A2 and a projection 12A is added to the connector 1A2 on the other end of the optical fiber. Projections 10B and 11B are added to a cap-side connector 5A1 in corresponding to the projections 10A and 11A and a projection 12B is added to another cap- side connector 1A1 in corresponding to the projection 12A. In such way, parts of the connectors other than their fitting parts are made different in shape by at least one or more. Therefore, when these optical fiber connectors are used, even an ordinary user can constitute easily and correctly an optical fiber using system while the commonness of the fitting of the connectors is maintained.