This short-distance radio communication system for a vehicle includes an in-vehicle unit having more than one antenna and a portable unit performing radio communications with the in-vehicle unit. The in-vehicle unit transmits a first burst together with a call signal for calling the portable unit through a first antenna, and transmits a second burst subsequent to the first burst through a second antenna. The portable unit receives the signals transmitted from the in-vehicle unit, measures respective received signal strength indicators from the first burst and the second burst contained in the received signals, and responds to the in-vehicle unit according to results of comparing the respective indicators with given thresholds.