Abstract: A motor is controlled so that it is not driven to further extend an antenna, even when a worm wheel receives a load from a drive cord and rotates to retract the antenna when the antenna is extended to its maximum length. The rotatable worm wheel for extending or contracting the drive cord is connected to a cam forming a motor operation controlling mechanism in which electricity is intermittently passed to the motor on the basis of the amount of rotation of the worm wheel. The cam can rotate through an angle of play .THETA. provided in the worm wheel to prevent the motor from being driven and the antenna from being extended further.