A communication apparatus for a vehicle can achieve an inexpensive technique for determining whether a portable unit exists inside or outside the vehicle without depending upon LF band radio waves through contactless communication using high frequency band radio waves by separating a reader function of a reader/writer into the portable unit and an on-board unit. The apparatus includes at least one ID transmitter installed on a vehicle and sends a transmitter ID possessed by itself in response to reception of a radio wave, an on-board unit installed on the vehicle for radiating a radio wave to the ID transmitter, a portable unit that receives the transmitter ID sent by the ID transmitter, and an ID communication section for sending the transmitter ID from the portable unit 3 to the on-board unit. The on-board unit determines the position of the portable unit based on the transmitter ID received from the portable unit.