PURPOSE: To obtain a water-proof rubber plug, which can be used for both two openings having diameters different form each other.

CONSTITUTION: A water-proof rubber plug main body 1 is made of soft rubber or soft synthetic resin having elasticity, and a wire insertion port 2 is formed along a center axis thereof, and plural grooves 2a are arranged in parallel with each other at a central part of the wire insertion port 2. The front peripheral surface of the water-proof rubber plug main body 1 is formed with an installation part 3 for a connecting terminal 3, and the rear peripheral surface of the installation part 3 is provided with a cylinder part 4 having a diameter larger than that of the installation part 3, and a cylinder part 5 having a diameter smaller than that of the cylinder part 4 is provided in the rear of the cylinder part 4. A hollow cylindrical outer cylinder part 7, which can be folded freely through a thin connection part 6 and which has the sufficient elastic reset force when it is compressed, is formed continuously to the peripheral surface at an intermediate position between the cylinder part 4 and the cylinder part 5. The outer cylinder part 7 is provided with a nearly circular or semi-circular annular projection 7a and a nearly circular or semi-circular annular projection 7b, which is positioned between the annular projection 7a and the connection part 6 and of which peripheral end is projected from the inner and the outer peripheral surfaces, at an opening edge thereof.