PURPOSE: To allocate plural slots efficiently by allocating again a slot based on the end of an optional call and reserving idle slots for plural slots used highest in same frequencies.

CONSTITUTION: In the radio communication system employing the TDMA radio system using plural carrier frequencies where one frame consists of plural slots to make communication between base stations 1a-1c and plural mobile stations 2a-2h, a control station 3 connects to other public network through a channel 4, and the base stations 1a-1c connect respectively to the control station 3 through channels 5a-5c. Then the base stations 1a-1c allocate a slot again in a timing when an optional call is finished and reserves an idle slot for plural slots with a highest use onto same carrier frequencies. Thus, even when a call requesting an idle slot of the plural slots with highest use comes, a connection delay or the like attended with re-allocation is avoided.