A control unit assigns time slots to terminal apparatuses, at regular intervals, over predetermined sub-channels among a plurality of sub-channels where the time slot is formed by frequency-multiplexing a plurality of sub-channels and frames each of which is formed by a plurality of time slots are contiguous. The control unit also arbitrarily assigns a plurality of time slots to terminal apparatuses, respectively, over the remaining sub-channels. An RF unit through an IF unit communicate with the terminal apparatuses assigned by the control unit.