The object of the present invention is to provide an IP multicast service system which can design an independent multicast group system in each network on the center side and the subscriber-side. In an IP multicast service system according to the present invention including: a center-side network configured by a plurality of multicast servers; and a subscriber-side network configured by a plurality of IP-STBs (Internet Protocol-Set Top Boxes) for receiving a multicast packet transmitted from the multicast servers and reproducing the multicast packet as an image, a group design on the basis of an independent group address system on the center-side network and on the subscriber-side network can be made by converting a group address of the multicast packet transmitted and received between the center-side network and the subscriber-side network.