Abstract: The portable device includes a control part for authenticating with an on-vehicle keyless control device by communication therewith to lock/unlock a door, a receiving part for receiving a radio signal from the control device, a transmitting part for outputting a response signal to the control device, a sender judging part for judging whether the radio signal comes from an interior transmitting antenna of the motor vehicle or from an exterior transmitting antenna thereof, and an output control part for changing an output power of the transmitting part based on a judgment result of the judging part so that the output power is decreased when the judging part judges the radio signal to come from the interior transmitting antenna, relative to an output power outputted when the judgment part judges the radio signal to come from the exterior transmitting antenna.