A method of fitting a rubber stopper for a water-proof connector terminal onto an end portion of an electric wire includes the steps of: successively feeding a group of rubber stoppers aligned in a row in a state wherein each of the rubber stoppers has its longitudinal axis extending vertically; transferring each of the rubber stoppers into a stopper housing portion; retaining the electric wire disposed on an imaginary line passing through the center of the stopper housing portion; and press-fitting the wire into the rubber stopper, whereby the rubber stoppers are successively and continuously fitted on the respective wires. Also disclosed is an apparatus which may be suitably employed to carry out the above-described method. The apparatus includes an aligned stopper successively feeding section, a stopper transfer section, a rotary conveyor section, and a wire press-fitting section. The apparatus is further provided with an automatic control circuit adapted to allow the respective operations of the above-described secitons to be sequentially interlocked with each other.