A broadcast retransmitting method, a broadcast retransmitting apparatus, a relay apparatus and a broadcast retransmitting system using the same, are provided. The broadcast retransmitting apparatus includes a demodulation unit which receives and demodulates a modulated broadcast, a modulation unit which modulates the demodulated broadcast, a spectrum sensing unit which detects an available band through spectrum sensing, and a control unit which retransmits the modulated broadcast to at least one surrounding apparatus. The broadcast retransmitting method includes receiving and demodulating a broadcast which has been modulated according to a first modulation method, and modulating the demodulated broadcast according to a second modulation method different from the first modulation method; detecting an available band from an idle band through spectrum sensing; and retransmitting the broadcast which has been modulated in the second modulation method to at least one surrounding apparatus using the detected available band.