1.  Technical Field
The present invention relates to a waterproof structure of a connector, and more specifically to a waterproof structure provided at a rear end portion of a connector for preventing water from entering the connector. 
2.  Description of the Related Art
An example of a prior art waterproof structure of a connector is disclosed in Japanese Published Unexamined (Kokai) Patent Application No.  3-122979, which is roughly composed of a rubber waterproof plug and a rubber plug cover, both for enclosing the rear end portion of a connector in cooperation with each other.  In this example, the rubber waterproof plug is formed with a sealing projection extending along the outer side surface of a square-shaped rubber plate so as to be air-tightly fitted to a rear end portion of a connector for prevention of water from entering the connector.  Further, the rubber waterproof plug is formed with a plurality of wire insertion holes through which wires extending from the connector are passed under airtight condition.  On the other hand, the rubber plug cover is so formed as to be fitted to the rear end portion of the connector so as to hold the rubber waterproof plug between the connector and the plug cover.  The rubber plug cover is also formed with a plurality of wire leading holes which communicate with the wire insertion holes of the rubber waterproof plug so that the wires can be led out from the inside of the connector to the outside thereof. 
In the prior art rubber waterproof plug as described above, however, there inevitably exists such a case that the wires are not passed through all the wire insertion holes.  In more detail, when the number of the wires led out of the connector is less than the number of the wire insertion holes of the rubber waterproof plug, some wire insertion holes are left open.  To close these open wire insertion holes, some hollow cylindrical projections are formed at such positions as to correspond to the non-used wire leading holes formed in the rubber plug cover, and in such a way as to be inserted into each of the non-used wire insertion holes formed in the rubber waterproof plug under airtight condition so that the non-used wire insertion holes (through which no wires are passed) can be closed air-tightly.  Further, when the positions of the non-used wire insertion holes change for each use, the hollow cylindrical projections are removed or punched out by use of a punch-out tool, so that the wires can be passed through both the wire insertion holes and the wire leading holes.  In other words, in the prior art rubber waterproof plug, the rubber plug cover is formed with some projections inserted into the non-used wire insertion holes of the rubber waterproof plug, In such a way that the projections can be removed in correspondence to the number of wires to be led out of the connector. 
In the prior art rubber waterproof structure, however, since the rubber plug cover is formed with some projections, there exists a problem in that the projections are easily damaged by an external force during transport, and thereby the sealing performance of the projections (i.e., the waterproof structure) of a connector deteriorates.  In addition, since it is necessary to locate the projections at the non-used wire insertion holes before inserting the projections thereinto, there exists another problem in that the mounting of the rubber plug cover to the waterproof plug is troublesome, and therefore it takes a relatively long time.  Furthermore, there exists another problem in that, since thin wall thickness projections must be formed so as to rise from the outer circumferential edge portions of the openings of the wire leading holes of the rubber plug cover, the shape of the rubber molding die is complicated and therefore costly.