There is provided a method for supporting a multicast service in an Ethernet passive optical network system. Since an Optical Network Unit (ONU) system monitors an IGMP (Internet Group Management Protocol) message and informs the monitored result of an Optical Line Terminal (OLT) system so that the OLT system creates L2 multicast forwarding table using a multicast address and ONU ID information in a PON port, the ONU system transmits the multicast packet only to a corresponding subscriber on the basis of the L2 multicast forwarding table when receiving a multicast packet. Therefore, it is possible to transfer a multicast packet only to a specific ONU system belonging to a corresponding multicast address group among a plurality of ONU systems.