Disclosed herein is a synchronization method for transmitting a DOCSIS upstream signal, which is performed by an RoIP terminal, the synchronization method including receiving a synchronization (SYNC) message periodically sent by a Cable Modem Termination System (CMTS) and thereby synchronizing a clock and generating a reference time; receiving a ranging request signal from a Cable Modem (CM); including information about a start time of an allocated ranging interval in the ranging request signal, converting the ranging request signal into an IP packet, and transmitting the IP packet to an RoIP headend device over an IP network in order to enable the RoIP headend device to convert the ranging request signal, delivered via the IP network, into an RF signal and transmit the RF signal to the CMTS; and converting a ranging response message delivered from the CMTS into an electric signal and transmitting the electric signal to the CM.