Provided are a method and an apparatus for transmitting retransmission data using an HARQ in a wireless communication system. Specifically, it is determined whether there is a radio resource for transmitting retransmission data. When the radio resource exists, a DCI format including a retransmission indicator field is configured. The retransmission indicator field indicates whether the retransmission data is general retransmission data or opportune retransmission data. The opportune retransmission data is retransmitted before an ACK/NACK signal for initial transmission data is received. The retransmission data is transmitted through the radio resource according to scheduling of a configured DCI format.