@ In a radio communication system comprising a fixed unit (10) carrying out a first scanning operation at a channel scanning period in a waiting state and a portable unit (11) intermittently put in a waiting state thereof into active and inactive for active and inactive durations, respectively, the portable unit comprises a power source circuit (30) for rendering a sum of the active and the inactive durations equal to the channel scanning period. In each active state, a frequency synthesizer (39) carries out a second scanning operation to successively scan the radio channels one by one in cooperation with the controller (38) to detect whether or not each of the radio channels is idle. The second scanning operation may proceeds either in the same order as the first scanning operation or in a reverse order rotative to the first scanning operation. Alternatively, the second scanning operation may be carried out in a phase different from the first scanning operation. In this event, either one of the fixed and the portable units that is delayed in phase may skip from an actual channel to an additional one.