This invention relates to a rubber plug for a waterproof connector used in a wire harness for an automobile, and an object thereof is to make the movement of the plug smooth when feeding the plug by a part feeder to an assembling line where the plug is connected to a metal terminal. According to the present invention, the rubber plug comprises a contact portion of a synthetic resin formed to extend axially from a seal portion of a rubber plug which extends continuously from a metal terminal-connection tubular portion having a wire passage internal bore, the seal portion being adapted to be in intimate contact with an inner wall of a connector housing. The rubber plug is moved with the sliding contact portion held in contact with a feed guide rail of a part feeder.